Birth defects

Causes of defects

Physical birth defects

Hereditary diseases and syndromes

Resources

Birth defects or congenital defects are those present at birth. They result from heredity, environmental influences, or maternal illness. Such defects range from the very minor, such as a dark spot or birthmark that may appear anywhere on the body, to more serious conditions that may result in marked disfigurement, impaired functioning, or decreased lifespan. A classification of structural defects can be as follows: Malformation (poor formation), deformation (due to fetal constraint that can result in damage (e.g., central nervous system damage or limb reduction) and disruption of previous normally formed structures (due to vascular damage, vascular exchange of necrotic debris).

A number of factors individually or in combination may cause birth defects. Heredity plays a major role in passing birth defects from one generation to the next. Inherited conditions are passed on when a baby receives a flawed gene from one or both parents. Conditions such as sickle cell anemia, color blindness, deafness, and extra digits on the hands or feet are hereditary. The condition may not appear in every generation, but the defective gene usually is passed on.

Causes of defects

Low birth weight deriving from a fetal growth restriction (FGR) is the most common birth defect, with one in every 15 babies being born at less than their ideal weight. A baby whose weight lies in lowest 10% of the normal population is designated as having a FGR. A baby who weighs 5 pounds, 8 ounces (2,500 g) at birth has a low birth weight. One weighing 3 pounds, 5 ounces (1,500 g) has a very low birth weight. A low–birth weight baby born after a normal gestation period is called a small-for-date or small-for-gestational-age baby.

A mother’s exposure to chemicals (such as mercury) or to radiation during the first three months of pregnancy may abnormally alter the baby’s growth or development. Poor maternal nutrition may also be a factor in a baby’s birth defect. A balanced and healthy diet is essential to the proper formation of the fetus because the developing baby receives all of its nutrition from its mother.

Prenatal fetal development may also be affected by disease that the mother contracts, especially those that occur during the first trimester (three months) of pregnancy. For example, if a pregnant woman catches rubella (German measles), the virus crosses the placenta and infects the fetus, where it interferes with normal metabolism and cell movement and can cause blindness (from cataracts), deafness, heart malformations, and mental retardation. The risk of the fetal damage resulting from maternal rubella infection is greatest during the first month of pregnancy (50%) and declines with each succeeding month.

It is especially important that the mother not smoke, consume alcohol, or take drugs while she is pregnant. Drinking large quantities of alcohol can cause fetal alcohol syndrome (FAS), a condition that produces small eyes and a short, upturned nose that is broad across the bridge, making the eyes appear farther apart than normal. These babies also are underweight at birth and do not catch up as time passes. They often have some degree of mental retardation and may exhibit behavior problems. A mother who takes illicit drugs such as heroin, crack, or cocaine will give birth to a baby who is already addicted. The addiction may not be fatal, but the newborn may undergo severe withdrawal, unless the addiction is revealed and carefully treated. Furthermore, some behavior problems and cognitive deficits are suspected to be associated with fetal drug exposure and addiction.

Even some therapeutic drugs taken by pregnant women have been shown to produce birth defects. The most notorious example is thalidomide, a mild sedative-hypnotic agent. During the 1950s women in more than 20 countries who took this drug gave birth to more than 7,000 severely deformed babies. The pattern of malformation seen in affected infants included phocomelia, in which long bones in the arms and legs are extremely short or missing altogether; polydactyly, which produces extra digits on hands and/or feet; syndactyly, in which digits are fused together; facial capillary hemangiomas, or “strawberry marks,” benign blood vessel tumors; hydrocephalus, an accumulation of cerebrospinal fluid in the skull, which can lead to brain damage; as well as renal, cadiovascular, ear, eye, and intestinal anomalies.

Physical birth defects

Clubfoot

Approximately 1 newborn out of every 735 has a form of clubfoot. In the most serious form, known as equinovarus, the foot is twisted inward and downward and the foot itself is cupped or flexed. If both feet are clubbed in this manner the toes point to each other rather than straight ahead. Often the heel cord or Achilles tendon is taut so that the foot cannot be straightened without surgery.

A milder and more common type of clubfoot is called calcaneal valgus, in which the foot is bent upward and outward in the same way that you would flex your foot at the ankle. Still other forms include talipes cavus in which the instep is abnormally elevated; talipes valgus in which the heel is turned outward, and talipes varus in which the heel is turned inward. Seriously deformed clubfeet require surgery to realign the bones and ligaments. Milder forms often can be cured by fitting the baby with corrective shoes to gradually move the bones back into alignment.

Cleft lip and cleft palate

Approximately 7,000 newborns (1 of every 930 births) are born with cleft lip and/or palate each year in the United States. These are conditions in which the fetus’s palatal plates remain split after the second month of gestation, when they would normally fuse. Although cleft lip and palate are two distinct anomalies, they frequently occur together. The cleft may involve only the upper lip, may extend into the palate, or may be located on the back of the palate. Cleft lip with or without cleft palate occurs in 60-75% of the cases. Twenty-five to 40% are isolated cleft palate. These failures are a consequence of the abnormal migration and proliferation of facial embryonic tissues called mesenchyme. The defect occurs most often among Asians and certain Native American groups, less frequently among whites, and least often among African Americans.

Approximately 25% of infants born with cleft palate inherit the trait from one or both parents. The cause for the other 75% remains unknown, but may be a combination of heredity, poor nutrition, drug use, or disease. Maternal smoking represents the most controversial association.

Surgery is especially important to correct palate defects. Feeding a baby with cleft palate is difficult because the food can pass through the palate into the nasal cavity, where it can be inhaled and cause choking. In the newborn, whose bones have not completely hardened, surgery is relatively simple. As the child ages, however, correction is more difficult and the child will require speech therapy.

Spina bifida

Spina bifida or open spine occurs once in 2,000 births in the United States. The term spina bifida means the spine is cleft, with an opening or space, in two parts. It is a type of neural tube defect that, after cardiac malformations, is the second-most prevalent neonatal anomaly in the United States. Spina bifida occurs when the edges of the spine, which should grow around the spinal cord, do not meet. An open area remains, which can mean that an area of the spinal cord (or, in the most severe cases, the entire spinal cord) are unprotected. The mildest form may be so slight that the defect does not have any effect on the child and is discovered by accident, usually when an x-ray is taken for another reason.

Spina bifida may present itself as a cyst, ranging in size from a walnut to a grapefruit, in which some parts of the meninges (layers of connective tissue covering the spinal cord), spinal cord, or both are contained. The lump can be removed surgically. In the most serious form, the lump or cyst has little skin or covering so spinal fluid may leak from it. Roots of spinal nerves are contained within the cyst, which may also be covered with sores. Infection is a serious risk until surgery has been performed and the area has healed. Unfortunately, this condition may leave the child’s legs partially or completely paralyzed and without feeling. Other associated problems may include bladder and bowel control.

Newborns with spina bifida often have an associated condition called hydrocephalus, which literally means water in the head. In this condition, cerebro-spinal fluid collects and is trapped in and around the brain. Brain damage and mental retardation can result if the fluid is not allowed to drain. This can be accomplished by implanting a special tube (called a shunt) leading from the brain down into a vein in the child’s neck or chest to allow the fluid to drain. Hydrocephalus also can occur in infants who do not have spina bifida.

Spina bifida can be diagnosed before birth by amniocentesis (by dosing the intra-amniotic alpha-feto protein) or ultrasound. The Spina Bifida Association urges women to take at least 400 mg of folic acid just before and throughout pregnancy; this can reduce the chance of spina bifida and other neural tube defects by as much as 70%.

Heart defects

Congenital heart defects occur in 1 of every 115 births in the United States. The defect may be so mild that it is not detected for some years, or it may be fatal. A baby with a heart defect may be born with a bluish tinge around its lips and on its fingers. This condition, called cyanosis, is a signal that the body is not receiving enough oxygen. The blue color may disappear shortly after birth, indicating that all is normal, or it may persist, indicating that further testing is needed to determine the nature of the heart defect.

A normal heart has four chambers; two upper, called the atria (singular: atrium) and two lower, called the ventricles. The right side of the heart receives deoxygenated blood that is returning from the body. This oxygen-poor blood arrives in the right atrium, where it is pumped into the right ventricle. The right ventricle sends the blood to the lungs, where it is picks up plenty of oxygen. This oxygen-rich blood then enters the left atrium and is pumped into the left ventricle, which pumps oxygen-rich blood through the aorta to all the organs and tissues of the body.

During fetal development, blood circulates differently, because the fetus’s blood does not need to flow through its lungs. It receives its oxygen from the mother through the placenta via the umbilical cord. Since the atria communicate during fetal life, blood rich in oxygen coming from inferior vena cava crosses the foramen ovale and flows into the left atrium, bypassing the lungs. (Eventually the foramen ovale is closed from the higher pressure generated at the left side after the lungs expand at birth.)

Another special shunt, the arterious duct connects the main pulmonary artery to the aorta. In this way, bloodflow that enters the right atrium also enters the right ventricle, then the main pulmonary artery, then into the ductus arteriosus, which connects to the aorta. In this way the vast majority of blood flow bypasses the lungs during fetal development. Normally the shunts should close at birth, and the baby’s lungs should deliver oxygen to the blood. Sometimes, however, the shunt does not close properly, and blood is not appropriately circulated through the lungs. When this occurs, surgery is required to close the shunt and restore normal circulation.

If undetected at birth, a heart defect may impair a child’s growth. He will be unable to exert the energy that other children do at play because he cannot supply sufficient oxygen to his body. He may become breathless at small amounts of exertion and may squat frequently because it is easier to breathe in that position.

Some minor defects may disappear over time as the child grows. A small hole in the wall between the left and right sides of the heart, which allows the mixing of oxygen-poor and -rich blood, for example, may spontaneously close over time. A larger defect requires surgical patching.

Some newborns may have only one upper chamber or only a single lower chamber of the heart. The aorta, where it begins at the heart, may be narrowed (stenosed) and impair the blood flow from the heart. Some heart valves may not function correctly, and occasionally the vessels may be transposed so that the aorta leads from the right side of the heart, delivering oxygen-poor blood to the organs and tissues.

These are only a few of the heart anomalies that can be present in newborn children. The heart is a complicated organ and its formation can be influenced by hereditary factors as well as by alcohol consumption or smoking. Fortunately, most heart defects correct themselves over time or can be corrected with surgery.

Other physical deformities

Physical defects in newborns are common. They can affect any of the bones or muscles in the body and may or may not be correctable. Among the more common are the presence of extra fingers or toes (polydactyly), which presents no health threat and can be corrected surgically. Similarly, webbed fingers and toes, a genetic disorder, seen in approximately one of every 1,700 to 2,000 births, can be treated surgically to create a normal appendage.

A more serious, though relatively rare, condition is called achondroplasia; this term means “without cartilageformation” and refers to the supposed lack of cartilage growth plates near the ends of a child’s bones. In fact, the plates are present, but grow poorly. Achondroplasia is a type of dwarfism. This genetic disorder is seen in one in 20,000 births and is one of the oldest-known birth defects. Ancient Egyptian art shows individuals with this condition. The cause is not known, nor is there a cure. Children who have this condition will be slow to walk and sit because of their short arms and legs, and this may be interpreted as mental retardation. However, they have normal intelligence.

Hereditary diseases and syndromes

In addition to physical deformities, certain diseases and syndromes also are passed to the infant through the parents’ genes. Some of these conditions can be controlled or cured while others are untreatable and fatal.

Sickle cell anemia

Sickle cell anemia is an inherited disease of the blood cells that occurs in about 1 of every 400 African Americans. An individual can be a carrier of sickle cell anemia, in which case he or she has the gene but does not show any active signs of the disease. If two carriers produce a child, however, it may have sickle cell anemia.

The disease gets its name because certain red blood cells assume a sickle shape and lodge in small blood vessels. This altered shape is a function of the hemoglobin molecule present in red blood cells. Two forms of hemoglobin make up these cells: hemoglobin A (Hb A) and hemoglobin B (Hb B). In individuals with sickle cell anemia, Hb B is instead produced as Hb S, a form of hemoglobin with a rigid, sickle shape that deforms the red blood cell. When the cell becomes wedged in a small blood vessel it prevents the flow of blood through the vessel and can initiate what is called a sickle cell crisis. The lack of blood flow to the tissues being blocked causes pain and inflammation of the oxygen-deprived tissue.

Abnormal red blood cells are removed from the circulatory system by the spleen, but removing large numbers of such cells can lead to anemia, an inadequate number of red blood cells. Unfortunately, the breakdown of abnormal red blood cells can in itself cause a serious condition in which excess iron, scavenged from the hemoglobin molecule, is deposited in tissues such as the heart and liver. So, although replacement of the destroyed red blood cells could be achieved with blood transfusion, the replacement cells will only add to the iron content of blood. There is no cure for sickle cell anemia, though scientists are learning how to better control it to prevent sickling of the blood cells.

Tay-Sachs disease

Tay-Sachs disease affects Ashkenazi Jews of eastern European origin, and is a condition that is fatal at an early age. A carrier will have a gene for Tay-Sachs disease and another gene that is normal. If two carriers have children, every pregnancy will have a 25% chance of producing a completely normal child; a 50% chance of producing a child who will carry the trait, but reveal no symptoms; and a 25% chance of producing a child who actually suffers from the disease.

Newborn Tay-Sachs children lack a blood enzyme called hexosaminidase A, which breaks down certain fats in the brain and nerve cells. When first born, the baby appears totally normal. However, over a short period of time, the brain cells become clogged with fatty deposits, and the child begins to lose functioning. As the disease progresses, the child will no longer be able to smile, crawl, or turn over, and will ultimately become blind and unaware of his surroundings. The child usually dies by the age of three or four years.

There is currently no cure for Tay-Sachs disease, although carriers can be detected by a simple blood test that measures the amount of hexosaminidase A. A carrier will have half the amount of the enzyme as a normal person, and two carriers can be counseled to explain the probability of producing an offspring with Tay-Sachs disease. Researchers are trying to find a way to provide sufficient levels of the missing enzyme in the newborn, or to find a suitable substitute that could be supplied as the child ages, much like insulin is used to treat diabetes. A more technologically advanced line of research is examining the possibility of transplanting a normal gene to replace the defective one in carriers.

KEY TERMS

Hemoglobin— The iron-containing substance within red blood cells that gives blood its red color and carries oxygen to the body’s cells.

In utero— In the uterus; prior to birth.

Phocomelia— from the Greek for “seal limbs,” a congenital defect in which long bones are extremely short or absent altogether, and the hands and feet are are flipper-like.

Placenta— The flat, plate-like organ of exchange between the blood of the mother and that of the embryo. It attaches to the wall of the uterus and provides nutrients and oxygen for the embryo and removes wastes from the embryo.

Down syndrome

One in every 800-1,000 babies is born with Down syndrome. Down syndrome babies may have eyes that slant upward, small ears that may turn over at the top, a small mouth and nose that also is flattened between the eyes (at the bridge). Mental retardation is present in varying degrees, but most Down syndrome children have only mild to moderate retardation. Generally these children walk, talk, dress themselves, and are toilet trained later than children with normal intelligence.

Down syndrome results when either the egg or the sperm that fertilizes it has an extra chromosome. Normally a human has 23 pairs of chromosomes, for a total of 46. An extra chromosome, specifically an extra number 21 chromosome, present when the egg is fertilized, leads to a baby with Down syndrome. Of course, if either parent has Down syndrome, the probability of passing the condition on to the offspring is increased. Also, parents who have had one Down syndrome child and mothers older than 35 years of age are at increased risk of having a Down syndrome baby. There is no cure, though many of these children can go on to attend school and hold jobs.

It should be apparent from this small sample, that some birth defects are hereditary, passed from parents to offspring; genetic therapy offers hope that this situation may change in the future. Other birth defects result from infections the mother contracts during pregnancy, or from maternal consumption of alcohol or drugs, use of tobacco, or exposure to radiation or chemicals during pregnancy. In some cases, these birth defects can be prevented through education or improved prenatal care.

See also Embryo and embryonic development; Genetics.

Resources

BOOKS

Nussbaum, R.L., Roderick R. McInnes, Huntington F. Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.

Rimoin, D.L. Emery and Rimoin’s Principles and Practice of Medical Genetics. London; New York: Churchill Livingstone, 2002.

Sadler, T.W., and Jan Langman. Langman’s Medical Embryology, 8th ed. Lippincott Williams & Wilkins Publishers, 2000.

OTHER

Nemours Foundation. “Birth Defects” <http://www.kidshealth.org/parent/system/ill/birth_defects.html> (accessed November 2, 2006).

Spina Bifida Association. “About Spina Bifida Overview” <http://www.sbaa.org/site/c.gpILKXOEJqG/b.2021049/k.C6B3/About_SB.htm> (accessed November 2, 2006).

Larry Blaser

Birth Defects

A birth defect is an anomaly that is congenital, or present from birth. Birth defects are the leading cause of infant mortality, causing 22 percent of all infant deaths. Approximately 3 to 4 percent of all live births are affected by a birth defect; the causes of most of them are unknown. Some birth defects are considered to be physical, while others are thought of as functional. Physical birth defects result in the malformation of a physical organ or limb, whereas functional birth defects are those that cause primarily functional, rather than physical, problems. Functional birth defects include mental retardation, congenital hearing loss, early-onset vision impairment, and numerous other health concerns. They may be caused by single gene mutations, or they may be due to polygenic or multifactorial inheritance.

Birth defects may be found in isolation or they may occur in combination in one child, as part of a larger syndrome. A syndrome is usually defined as the presence of three or more birth defects due to one underlying cause, that characterize a particular disease or condition. An example of this is Down syndrome.

Various Causes, Various Treatments

There are several etiologies of birth defects, including single gene mutations, polygenic and multifactorial conditions, chromosomal abnormalities, and teratogens, which cause growth or developmental abnormalities. These etiologies may cause both physical and functional birth defects. The diagnosis, treatment, and management of birth defects often involves a team of professionals and specialists. Among these specialists are clinical geneticists, medical geneticists, and genetic counselors. These are all genetic service providers who are trained to help make a diagnosis and identify whether a birth defect is isolated or part of a syndrome. An accurate diagnosis is of utmost importance in being able to treat the condition and anticipate any future health concerns that may arise.

Single-Gene Mutations

Single-gene mutations are defects or changes in genes that may be passed on from generation to generation. The most common forms of inheritance include autosomal dominant, autosomal recessive, X-linked recessive, and new dominant mutations. Single-gene mutations may result in both structural and functional birth defects. These defects may be found in isolation or as part of a known syndrome and may display phenotypic variation, in which the same mutant gene leads to variable clinical problems. This often occurs with dominant mutations such as Marfan syndrome. A genetic disorder may also display reduced penetrance, wherein not everyone with the genetic mutation will have the disorder or trait. These are also known as "silent" disorders.

Autosomal Dominant Disorders

An example of an autosomal dominant disorder is achondroplasia, the most common form of short-limbed dwarfism in humans. Achondroplasia displays complete penetrance (everyone with the genetic defect also has the disorder), and it occurs in 1 out of 25,000 births. Most cases are sporadic rather than inherited.

Achondroplasia is a growth disorder caused by a mutation of the gene that encodes the fibroblast growth factor receptor 3 (FGFR3 ), and it is characterized by short limbs, malformed hands, a disproportionately large head, and abnormal facial features. Medical problems are due to abnormally configured bones and related structures, leading to hydrocephalus, problems of the spine, frequent sinus and ear infections, and orthopedic problems. If one parent is affected with achondroplasia, there is a 50 percent risk that an offspring will also be affected. If both parents are affected, there is a 25 percent chance that an offspring will inherit two gene copies and develop severe, life-threatening features. Two known mutations in FGFR3 account for 98 percent of all achondroplasia cases. This makes early identification, even prenatal diagnosis, relatively easy.

Another autosomal dominant condition is Marfan syndrome, which results from a defect in the synthesis, secretion, or utilization of the protein fibrillin, an important component of connective tissue throughout the body. The gene for Marfan syndrome is fibrillin 1 (FBR1 ). Marfan syndrome features are variable, including cardiovascular, skeletal, and ocular defects.

Marfan syndrome's most serious medical complication is the risk of sudden death from aortic dissection, a tear in the inner wall of the major artery leading from the heart. Approximately 75 percent of individuals with Marfan syndrome have a family history of the disease, with the rest occurring as new mutations. Because the syndrome is autosomally dominant, affected indviduals have a 50 percent risk of passing the mutated gene to their off-spring. The condition has full penetrance; therefore, all individuals who inherit this mutation will express some features of Marfan syndrome.

Functional Birth Defects

Fragile X syndrome is the most common cause of inherited mental retardation, occurring in one out of 1,000 births. It is caused by expansion of a "triplet repeat" section of nucleotides in the FMR-1 gene on the X chromosome. Triplet repeats are three-base-pair sequences in a gene that are abnormally repeated, sometimes dozens or even hundreds of times, causing abnormal protein sequence and structure. Because it is carried on the X chromosome, it affects males more often than females. In males, an FMR-1 gene with greater than 200 repeats is always associated with the syndrome. Inactivated FMR-1 gene causes impaired mental function. The FMR-1 protein is thought to help shape the connections between neurons that underlie learning and memory. Affected individuals may also have large testes, abnormal facial features, seizures, and emotional and behavior problems. DNA testing allows for detection of carriers as well as affected individuals, enabling the use of genetic counseling and prenatal testing.

Another common functional genetic disorder is hearing loss, which can result from a defect in any one of more than fifty different genes. One gene, connexin 26, may be responsible for a large portion of inherited hearing loss. Some forms of congenital hearing loss may be due to prenatal exposure to infectious agents such as rubella. Genetic screening or screening for hearing loss at birth may be the most important test for hearing impairment yet to be developed, as early recognition and treatment can lead to dramatic improvements in hearing and, consequently, in the development of language in early childhood.

Other genetic disorders that cause functional birth defects include those involved in various aspects of the immune system. The most severe form is severe combined immune deficiency (SCID), in which a major type of immune cell, the lymphocyte, is absent. People with SCID suffer life-threatening infections beginning in infancy and may require complete physical isolation. This was the case for David Vetter, who became known to the world as the "bubble boy."

Multifactorial and Polygenic Inheritance

Many traits and diseases are caused by the interaction of inherited genes and the environment. These are known as "multifactorial" traits. While all genes interact with the environment, the impact of the environment in multifactorial traits and diseases is usually greater than in single-gene traits and diseases. Prenatal environmental influences are inevitably filtered through the maternal-placental system and include factors such as infections, drugs, tobacco or alcohol use, diabetes, and industrial toxins.

Polygenic traits and diseases are due to the cumulative effect of multiple genes, working together. Many congenital birth defects are thought to be multifactorial, such as pyloric stenosis (narrowing of the passage from stomach to intestine), cleft lip and palate, clubfoot, and neural tube defects. When found as isolated birth defects, these conditions are thought to be explained by a "multifactorial threshold model."

The multifactorial threshold model assumes the gene defects for multifactorial traits are normally distributed within the population. This means that almost everyone has some genes involved with these conditions, with most individuals having too few of them to cause disease. Individuals will not become affected with the condition unless they have a genetic liability that is significant enough to push them past the threshold, moving them out of the unaffected range and into the affected range (Figure 1).

Examples of Multifactorial and Polygenic Effects

Cleft lip with or without cleft palate (CL/P) is a heterogeneous disorder (those children affected may have somewhat different abnormalities) occurring in 1 out of 1,000 births. Some CL/P cases occur as isolated birth defects, while others occur as part of a larger syndrome. The majority of CL/P cases are associated with multifactorial inheritance. The risk to relatives of affected individuals can be anywhere from 0.5 to 15 percent, depending on the severity of the clefting and the degree of relationship to the affected individual, with risks highest for first-degree relatives. Some unique cases of CL/P may be associated with genetic syndromes that are due to single-gene mutations or chromosomal abnormalities.

Clubfoot is another primarily multifactorial defect and occurs in 1 out of 10,000 Caucasian newborns. The estimated risk to relatives of inheriting this defect is between 2 to 20 percent, depending upon the family history. Clubfoot can also have genetic causes such as chromosomal abnormalities or single-gene disorders, or it may have an environmental origin, such as problems caused by amniotic fluid or structural abnormalities of the uterus that restrict fetal growth and mobility. Clubfoot can also be due to autosomal recessive as well as autosomal dominant inheritance, and it may also occur as part of a larger syndrome.

Another class of multifactorial disorders is known as neural tube defects (NTDs). The neural tube is the embryonic structure that develops into the brain and spinal cord. Failure of the neural tube to close, which normally occurs during in the fourth week of gestation, results in an NTD, usually spina bifida or anencephaly. Spina bifida ("open spine") is a defect of the spine. The most common form of spina bifida causes some degree of leg paralysis, impaired bladder and bowel control, and sometimes mental retardation. Anencephaly is a rapidly fatal condition in which a baby is born with a severely underdeveloped brain and skull.

While most NTDs are inherited as multifactorial disorders, a few result from single-gene disorders, chromosomal abnormalities, or teratogens. NTDs currently have an incidence of 1 per 2,000 births. This rate has fallen dramatically over the past thirty years, due to the remarkable effects of NTD-prevention efforts. Maternal deficiency in folate (a B vitamin) greatly increases the risk of NTDs, but taking multivitamins containing folic acid before conception and early in pregnancy is highly effective in preventing these disorders. High doses of folic acid are needed to help protect the fetuses of women with pregnancies previously affected by NTDs, and for those who need to take certain medications that interfere with folate metabolism .

Chromosome Disorders

Humans normally have twenty-two pairs of autosomes and two sex chromosomes, XX or XY, making forty-six chromosomes in total. Chromosomal abnormalities occur in about 0.5 percent of all live births and are usually due to an abnormal number of chromosomes. These are nearly always an addition or deletion of a single autosome or sex chromosome in a pair. One extra copy of a chromosome is called a trisomy, while one missing copy of a chromosome is called a monosomy. Sometimes only a segment of a chromosome is duplicated or lost.

Chromosomal disorders are diagnosed by karyotype analysis and can be done on adults by testing blood, skin, or other tissue. Karyotypes can also be performed on a fetus through specialized testing such as amniocentesis. Prenatal maternal blood tests are routinely used to screen for some trisomies, though accurate diagnosis requires fetal karyotyping.

Chromosomal abnormalities can occur in offspring of mothers of all ages, but the frequencies of these disorders increase with maternal age, rising exponentially after the maternal age of thirty-five. Advanced paternal age has far less impact. Chromosomal abnormalities can result in either physical or functional birth defects. The severity of these birth defects is highly variable and depends upon the exact chromosome problem.

Chromosomal defects include such problems as Down syndrome, Kline-felter's syndrome, and Turner's syndrome. The majority of Down syndrome cases are due to an extra chromosome 21. Trisomy 21 usually occurs as an isolated event within a family. It results in characteristic facial features, lax muscle tone, cardiac and intestinal anomalies, and mild or moderate mental retardation. Additional medical complications may also include recurrent ear and respiratory tract infections, vision problems, hearing difficulties, and short stature.

Klinefelter's syndrome is a sex chromosome abnormality that occurs in 1 of 600 males, with a karyotype of 47. Individuals with Klinefelter's syndrome possess an extra X chromosome: XXY. Clinical characteristics are variable and include some learning and developmental disabilities, hypogonadism , small testes, and gynecomastia occuring in puberty. The condition can be managed by administering testosterone supplements beginning in adolescence. As with some other sex chromosome abnormalities, adults with Klinefelter's syndrome are usually infertile.

Turner's syndrome is another sex chromosome disorder, with a karyotype of 45. In this condition, one X chromosome is missing. Turner's syndrome occurs in one out of 4,000 live births. Most females with Turner's syndrome are short and have webbing of the neck, a broad chest, and a lack of ovarian development, with a consequent lack of pubertal development and infertility. Female hormone therapy is often used to induce breast development and menstruation. The majority of conceptions resulting in a fetus with this condition end in a miscarriage, as Turner's syndrome is highly lethal in early fetal development.

Teratogen Exposure

A teratogen is any agent that can cause birth defects if a fetus is exposed to it. Teratogens are usually drugs or infectious agents such as bacteria or viruses, and can affect a fetus from as early as the first few weeks after conception through the second trimester. For this reason, ideally, women should avoid all medications during pregnancy. However, in some circumstances there are medical risks and benefits that must be weighed, particularly if a medication is important to the health of the prospective mother.

To properly assess the potential danger of a teratogen, information is required about its effect on embryonic development, its ease of passage across the placenta, and the dosage and timing of fetal exposure to the teratogen. There are, however, limitations on the ability of doctors to predict the risk of birth defects arising from fetal exposure to any particular drug because of the lack of information on the effects of multiple medication use and possible drug interactions, the inability to control for other exposures women may have during pregnancy, and the unique genetic susceptibilities of each person. In addition, there are limited clinical studies that address this problem.

Risks for birth defects or adverse pregnancy outcomes associated with any type of exposure are in addition to the 3 percent background risk for birth defects in all pregnancies. There is no evidence linking paternal exposures to teratogenicity for the developing fetus, though exposure to some agents can reduce male fertility.

One well-documented teratogen is the drug thalidomide, which was taken by tens of thousands of women in the 1950s and early 1960s to treat nausea during pregnancy, before its potent teratogenic effects were recognized. Even a single dose caused severe birth defects, including amelia (absence of limbs), phocomelia (short limbs), incomplete or absent bone growth, ear and eye abnormalities, congenital heart defects, and others.

Another potent teratogen is a substance called isotretinoin, marketed under the brand name of Accutane, used to treat severe acne. Birth defects following prenatal exposure include serious central nervous system defects such as hydrocephalus, microcephaly , and mental retardation, as well as cleft lip and palate and anomalies of cardiovascular, limb, eye, and other systems. For women who have taken this substance, it is recommended to delay pregnancy at least one month after they have stopped using it.

Like medications, "recreational" drugs such as alcohol and cocaine can act as teratogenic agents. Fetal alcohol syndrome (FAS) and fetal alcohol effects (FAE) are the most common, completely preventable, and potentially devastating disorders caused by alcohol use during pregnancy. FAS is one of the leading causes of mental disabilities in children. Fetal exposure to alcohol creates additional problems in children born with FAS/FAE, such as characteristic facial features, growth retardation, central nervous system difficulties, learning disabilities, and behavioral problems. No amount of alcohol is thought to be safe during pregnancy; however, some of its effects may be prevented by stopping the exposure during or shortly after the first trimester.

Cocaine use during pregnancy is known to increase the risk of miscarriages and premature labor and delivery. Disturbances in the behavior of exposed newborns have been reported, such as irritability, irregular sleeping patterns, muscular rigidity, and poor feeding. Some birth defects associated with the use of this drug include urinary and genital malformations, as well as defects of the limbs, intestines, and the skull.

Conditions arising from infectious teratogenic agents include toxoplasmosis, syphilis, and rubella. In each of these cases, the mother is exposed to the infectious agent, then transmits it to the fetus. In toxoplasmosis, the parasite Toxoplasma gondii can be transmitted from cats to humans through contact with cat feces (cleaning litter box or gardening), or through consumption of undercooked meats, poorly washed fruits and vegetables, goat's milk, or raw eggs. Mother-to-fetus transmission is more likely if maternal infection occurs in the last few weeks before delivery, but early fetal exposure is generally associated with greater severity of defects in the child. Overall, 20 to 30 percent of untreated, infected newborns have birth defects, including seizures, microcephaly, and other severe effects on the nervous system. Treatment of the mother with antibiotics during pregnancy is safe for the fetus, and significantly reduces the likelihood of fetal infection.

Syphilis is an infection caused by the spirochete Treponema pallidum. This bacterium crosses the placenta and may result in fetal infection. If untreated, the pregnancy may end in miscarriage, stillbirth, or neonatal death. Signs of congenital infection include jaundice, joint swelling, rash, anemia, and characteristic defects of bone and teeth. Maternal treatment of this condition may help prevent the transmission to the fetus and its ill effects.

Rubella is the scientific name for the disease commonly known as German measles. Congenital rubella syndrome (CRS) results from the exposure of an unprotected pregnant woman to the rubella virus, and can lead to major birth defects, including serious malformations of the heart, blindness, deafness, and mental retardation. CRS has been virtually eradicated in the United States because of the near-universal vaccination against rubella, now part of the standard childhood immunizations program. Unfortunately, this vastly improved situation is not as common in much of the rest of the world.

Maternal Conditions

Birth defects can also result from physical conditions affecting the health of the mother. One common maternal condition associated with birth defects is diabetes mellitus, a multifactorial disorder. Mothers with diabetes have a two-to three-fold times greater risk of having a child with birth defects than the general population if their condition is not well controlled. However, good glucose control has been shown to correlate with a decreased risk of congenital malformations. Characteristic diabetic malformations include cardiovascular, craniofacial, genitourinary, gastrointestinal, and neurological abnormalities. The risk that the child born of a mother with diabetes mellitis will also develop diabetes as an adult is 1 to 3 percent.

Another maternal condition giving rise to birth defects is maternal phenylketonuria (PKU). This is an autosomal recessive disorder in which an enzyme called phenylalanine hydroxylase is defective. This enzyme normally converts a substance in the blood called phenylalanine to another substance called tyrosine. As a result, phenylalanine levels are high, resulting in mental retardation, microencephaly, growth retardation, cardiac problems, seizures, vomiting, and hyperactivity. Other traits associated with PKU are fair hair and skin and blue eyes. PKU can be effectively managed through changes in the diet, and women who have appropriately managed their diet can have pregnancies with healthy offspring.

see also Chromosomal Aberrations; Clinical Geneticist; Complex Traits; Diabetes; Down Syndrome; Fragile X Syndrome; Genetic Counseling; Growth Disorders; Severe Combined Immune Deficiency; Triplet Repeat Disease.

Nancy S. Green

and Terri Creeden

Bibliography

Batshaw, Mark L. When Your Child Has a Disability: The Complete Sourcebook of Daily and Medical Care. Baltimore, MD: Paul H. Brooks Publishing, 2001.

"Leading Categories of Birth Defects." March of Dimes Perinatal Data Center, 2000.

Internet Resource

National Organization of Rare Disorders. <http://www.rarediseases.org>.

The average IQ of an individual with Down syndrome is 55. Most tests cite an IQ of 100 as "average."

Birth defects

Birth defects or congenital defects are present at birth. They result from heredity, environmental influences, or maternal illness. Such defects range from the very minor, such as a dark spot or birthmark that may appear anywhere on the body, to more serious conditions that may result in marked disfigurement, impaired functioning, or decreased lifespan.

A number of factors individually or in combination may cause birth defects. Heredity plays a major role in passing birth defects from one generation to the next. Inherited conditions are passed on when a baby receives a flawed gene from one or both parents. Conditions such as sickle cell anemia , color blindness , deafness, and extra digits on the hands or feet are hereditary. The condition may not appear in every generation, but the defective gene usually is passed on. A classification of structural defect can be as follows: Malformation (poor formation), deformation (due to fetal constraint that can result in damage (e.g., central nervous system damage or limb reduction) and disruption of previous normally formed structures (due to vascular damage, vascular exchange of necrotic debris).

Causes of defects

Low birth weight deriving from a fetal growth restriction (FGR) is the most common birth defect, with one in every 15 babies being born at less than their ideal weight. A baby whose weight lies in lowest 10% of the normal population is designated as having a FGR. At term of pregnancy, a baby who weighs 5 lb, 8 oz (2,500 g) at birth has a low birth weight. One who is born weighing 3 lb, 5 oz (1,500 g) has a very low birth weight. A low birth weight baby born after a normal gestation period is called a small-for-date or small-for-gestational-age baby.

Exposure of the mother to chemicals such as mercury or to radiation during the first three months of pregnancy may result in an abnormal alteration in the growth or development of the fetus. The mother's diet may also be a factor in her baby's birth defect. A balanced and healthy diet is essential to the proper formation of the fetus because the developing baby receives all of its nutrition from the mother.

Prenatal development of the fetus may also be affected by disease that the mother contracts, especially those that occur during the first trimester (three months) of pregnancy. For example, if a pregnant woman catches rubella, the virus crosses the placenta and infects the fetus. In the fetus, the virus interferes with normal metabolism and cell movement and can cause blindness (from cataracts), deafness, heart malformations, and mental retardation. The risk of the fetal damage resulting from maternal rubella infection is greatest during the first month of pregnancy (50%) and declines with each succeeding month.

It is especially important that the mother not smoke, consume alcohol, or take drugs while she is pregnant. Drinking alcohol heavily can result in fetal alcohol syndrome (FAS), a condition that is physically apparent. FAS newborns have small eyes and a short, upturned nose that is broad across the bridge, making the eyes appear farther apart than normal. These babies also are underweight at birth and do not catch up as time passes. They often have some degree of mental retardation and may exhibit behavior problems. A mother who continues to take illicit drugs such as heroin, crack, or cocaine will have a baby who is already addicted. The addiction may not be fatal, but the newborn may undergo severe withdrawal, unless the addiction is revealed and carefully treated. Furthermore, some behavior problems/cognitive deficits are suspected to be associated with fetal drug exposure and addiction.

Some therapeutic drugs taken by pregnant women have also been shown to produce birth defects. The most notorious example is thalidomide , a mild sedative-hypnotic agent. During the 1950s women in more than 20 countries who had taken this drug gave birth to more than 7,000 severely deformed babies. The pattern of malformation seen in affected infants included phocomelia, polydactyly, syndactyly, facial capillar hemangiomas, hydrocephalus , renal anomalies, cadiovascular anomalies, ear and eye defects, and intestinal anomalies. The principal defect these children suffered is phocomelia, characterized by extremely short limbs often with no fingers or toes.

Physical birth defects

Clubfoot

Approximately one newborn out of every 735 has a form of clubfoot. In the most serious form, known as equinovarus, the foot is twisted inward and downward and the foot itself is cupped or flexed. If both feet are clubbed in this manner the toes point to each other rather than straight ahead. Often the heel cord or Achilles tendon is taut so that the foot cannot be straightened without surgery .

A milder and more common type of clubfoot is called calcaneal valgus, in which the foot is bent upward and outward in the same way that you would flex your foot at the ankle. Still other forms include the talipes cavus in which the instep is abnormally elevated; talipes valgus in which the heel is turned outward, and talipes varus in which the heel is turned inward.

The seriously deformed clubfoot requires surgery to realign the bones and ligaments. The milder forms often can be cured by fitting the baby with corrective shoes to gradually move the bones back into alignment.

Cleft lip and cleft palate

Approximately 7,000 newborns (one of every 930 births) are born with cleft lip and/or cleft palate each year in the United States. Cleft lip and palate describe a condition in which a split remains in the lip and roof of the mouth. Although cleft lip and palate are two distinct anomalies, they frequently occur together. Cleft lip with or without cleft palate occurs in 60-75% of the cases. twenty-five to forty percent are isolated cleft palate. During growth in utero (in the womb) the lip or palate, which develop from the edges toward the middle, fail to grow together. Such a failure is a consequence of the abnormal migration and proliferation of facial embryonic tissues called mesenchyme. The defect occurs most often among Asians and certain Native American groups, less frequently among whites, and least often among African Americans.

Approximately 25% of infants born with cleft palate have inherited the trait from one or both parents. The cause for the other 75% remains unknown, but may be a combination of heredity, poor nutrition, use of drugs, or a disease the mother contracted while pregnant. Maternal smoking represents the most controversial association. The cleft may involve only the upper lip, may extend into the palate, or may be located on the back of the palate.

Surgery is especially important to correct the defect in the palate. Feeding a baby with cleft palate is difficult because the food can pass through the palate into the nasal cavity and may be inhaled and cause choking. In the newborn, whose bones have not completely hardened, surgery is relatively simple. As the child ages, however, surgical correction is more difficult and the child will require speech therapy.

Spina bifida

Spina bifida or open spine occurs once in 2,000 births in the United States. It belongs to a group of defects known as neural tube defects that are the second most prevalent neonatal anomaly in the United States after cardiac malformations. It occurs when the edges of the spine that should grow around the spinal cord do not meet. An open area remains, which can mean that an area of the spinal cord (or the entire spinal cord, in the most severe cases) are unprotected. The mildest form of spina bifida may be so slight that the defect does not have any effect on the child and is discovered by accident, usually when an x ray is taken for another reason. The term spinal bifida means the spine is cleft, having an opening or space, in two parts.

Spina bifida may present itself as a cyst, ranging in size from a walnut to a grapefruit, in which some parts of the meninges (layers of connective tissue covering the spinal cord), spinal cord, or both are contained. The lump can be removed surgically. In the most serious form, the lump or cyst has little skin or covering so spinal fluid may leak from it. Roots of the spinal nerves are contained within the cyst and the cyst may be covered with sores. Infection is a serious risk until surgery has been performed and the area has healed. Unfortunately, this condition may leave the child's legs partially or completely paralyzed and without feeling. Other associated problems may include control of the bowels and bladder.

Newborns with spina bifida often have an associated condition called hydrocephalus, which literally means water in the head. In this condition, cerebrospinal fluid collects in and around the brain and will not drain. Mental retardation can result if the fluid is not drained regularly. This can be accomplished by implanting a special tube (called a shunt) leading from the brain down into a vein in the child's neck or into the child's chest to allow the fluid to drain harmlessly. Hydrocephaly also can occur in infants who do not have spina bifida. The cause of spina bifida is not known, nor is any means of prevention. It can be diagnosed before birth by amniocentesis (by dosing the intra-amniotic alpha feto protein) or ultrasound. The risk of having a baby with spina bifida or other associated defects seems to be reduced if a woman takes at least 400 mg of folic acid just before and throughout pregnancy.

Heart defects

Congenital heart defects occur in one of every 115 births in the United States. The defect may be so mild that it is not detected for some years or it may be fatal. A baby with a heart defect may be born showing a bluish tinge around the lips and on the fingers. This condition, called cyanosis, is a signal that the body is not receiving enough oxygen . The blue color may disappear shortly after birth, indicating that all is normal, or it may persist, indicating that further testing is needed to determine the nature of the heart defect.

A normal heart has four chambers; two upper, called the atria (singular: atrium) and two lower called the ventricles. The right heart receives the blood that is returning from the body, and has been depleted of oxygen. This oxygen-poor blood arrives in the right atrium, where it is pumped into the right ventricle. The right ventricle sends oxygen-poor blood to the lungs, where it is exposed to and picks up plenty of oxygen again. This oxygen-rich blood enters the left atrium and is then pumped into the left ventricle. The left ventricle pumps oxygen-rich blood through the aorta to all the organs and tissues of the body.

During fetal development, blood circulation occurs differently, because the fetus' blood does not need to flow through its lungs. It receives its oxygen from the mother through the placenta via the umbilical cord. Since the atria communicate during fetal life, blood rich in oxygen coming from inferior vena cava crosses the foramen ovale and into the left atrium bypassing the lungs (eventually the foramen ovale is closed from the higher pressure generated at the left side after the lunds expand at birth). Another special shunt, the arterious duct connects the main pulmonary artery to the aorta. In such a way, the blood flow that does enter the right atrium enters the right ventricle, then the main pulmonary artery, then enters the ductus arteriosus which connects to the aorta. In this way the vast majority of blood flow bypasses the lungs during development of the fetus. Normally the shunts should close at birth. After birth, blood should begin to circulate through the lungs for the first time, because the newborn baby's lungs are now responsible for delivering oxygen to the blood. Sometimes, however, the shunt does not close properly, and blood is not appropriately circulated through the lungs. When this occurs, surgery is required to close the shunt and restore normal circulation.

If it is undetected at birth, a heart defect may impair the growth of a child. He will be unable to exert the energy that other children do at play because he cannot supply sufficient oxygen to his body. He may become breathless at small amounts of exertion and may squat frequently because it is easier to breathe in that position.

Some minor defects may disappear over time as the child grows. A small hole in the wall between the left and right sides of the heart, which causes symptoms by allowing the mixing of oxygen-poor and oxygen-rich blood, for example, may spontaneously close over time. A larger defect will require surgical patching.

Some newborns may have only one upper chamber or only a single lower chamber of the heart. The aorta, where it begins at the heart, may be narrowed (stenosed) and impair the flow of blood from the heart. Some of the heart valves may not function correctly and occasionally the vessels of the heart may be transposed so that the aorta leads from the right side of the heart, delivering oxygen-poor blood to the organs and tissues.

These are only a few of the heart anomalies that can be present in the newborn. The heart is a complicated organ and its formation can be influenced by hereditary factors as well as by alcohol consumption or smoking. Fortunately, most heart defects correct themselves over time or can be corrected with surgery.

Other physical deformities

Physical defects in newborns are common. They can affect any of the bones or muscles in the body and may or may not be correctable. Among the more common are the presence of extra fingers or toes (polydactyly), which presents no health threat and can be corrected surgically. Similarly, webbed fingers and toes, a genetic disorder, seen in approximately one of every 1,700 to 2,000 births, can be treated surgically to resemble a normal appendage.

A more serious, though relatively rare, condition is called achondroplasia; this term means without cartilage formation and refers to the supposed lack of cartilage growth plates near the ends of a child's bones. In fact, the plates are present, but grow poorly. Achondroplasia is a type of dwarfism. This genetic disorder of bone growth is seen in one in 20,000 births and is one of the oldest known birth defects. Ancient Egyptian art shows individuals with this condition.

The cause of achondroplasia is not known, nor is there a cure. The child who has this condition will be slow at walking and sitting because of his short arms and legs, and this may be interpreted as mental retardation. However, these individuals have normal intelligence.

Hereditary diseases and syndromes

In addition to physical deformities, certain diseases and syndromes also are passed to the infant through the parents' genes. Some of these conditions can be controlled or treated while others are untreatable and fatal.

Sickle cell anemia

Sickle cell anemia is an inherited disease of the blood cells that occurs in about one of every 400 African Americans. An individual can be a carrier of sickle cell anemia, in which case he or she has the gene but does not show any active signs of the disease. If two carriers become parents, however, some of their children may have sickle cell anemia.

The disease gets its name because certain red blood cells assume a sickle shape and lodge in small blood vessels. This altered shape is a function of the hemoglobin molecule present in red blood cells. Two forms of hemoglobin make up these cells: hemoglobin A (Hb A) and hemoglobin B (Hb B). In individuals with sickle cell anemia, Hb B is instead produced as Hb S, a form of hemoglobin with a rigid, sickle shape that deforms the red blood cell. When the cell becomes wedged in a small blood vessel it prevents the flow of blood through the vessel and can initiate what is called a sickle cell crisis. The lack of blood flow to the tissues being blocked causes pain and inflammation of the oxygen-deprived tissue .

Abnormal red blood cells are removed from the circulatory system by the spleen, but removal of large numbers of such cells can lead to anemia, a lack of adequate numbers red blood cells. Unfortunately, the breakdown of abnormal red blood cells can in itself cause a serious condition in which excess iron , scavenged from the hemoglobin molecule, is deposited in tissues such as the heart and liver. So, although replacement of the destroyed red blood cells could be achieved with blood transfusion, the replacement cells will only add to the iron content of blood. There is no cure for sickle cell anemia, though scientists are learning how to better control it to prevent sickling of the blood cells.

Tay-Sachs disease

Tay-Sachs disease affects Jews of eastern European origin, the Ashkenazi Jews, and is a condition that is fatal at an early age. A carrier of the disease will have a gene for Tay-Sachs disease and another gene that is normal. If two carriers have children, every pregnancy will have a 25% chance of producing a completely normal child; a 50% chance of producing a child who will carry the trait, but reveal no symptoms; and a 25% chance of producing a child who actually suffers from the disease.

The newborn Tay-Sachs child lacks a blood enzyme called hexosaminidase A, which breaks down certain fats in the brain and nerve cells. When first born, the baby appears totally normal. However, over a short period of time, the brain cells become clogged with fatty deposits, and the child begins to lose functioning. As the disease progresses, the child will no longer be able to smile, crawl, or turn over, and will ultimately become blind and unaware of his surroundings. Usually the child dies by the age of three or four years.

There is currently no cure for Tay-Sachs disease, although carriers can be detected by a simple blood test that measures the amount of hexosaminidase A. A carrier will have half the amount of the enzyme as a normal person, and two carriers can be counseled to explain the probability of producing an offspring with Tay-Sachs disease. Researchers are trying to find a way to provide sufficient levels of the missing enzyme in the newborn, or to find a suitable substitute that could be supplied as the child ages, much like insulin is used to treat diabetes. A more technologically advanced line of research is examining the possibility of transplanting a normal gene to replace the defective one in carriers.

Down syndrome

One in every 800-1,000 babies is born with Down syndrome . Down syndrome babies may have eyes that slant upward, small ears that may turn over at the top, a small mouth and nose that also is flattened between the eyes (at the bridge). Mental retardation is present in varying degrees, but most Down's syndrome children have only mild to moderate retardation. Generally these children walk, talk, dress themselves, and are toilet trained later than children with normal intelligence.

Down syndrome results when either the egg or the sperm that fertilizes it has an extra chromosome . Normally a human has 23 pairs of chromosomes, for a total of 46. An extra chromosome, specifically an extra number 21 chromosome, present when the egg is fertilized, leads to a baby with Down syndrome. Of course, if either parent has Down syndrome, the probability of passing the condition on to the offspring is increased. Also, parents who have had one Down syndrome child and mothers older than 35 years of age are at increased risk of having a Down syndrome baby. There is no cure, though many of these children can go on to attend school and hold jobs as do unaffected individuals.

It should be apparent from this small sample , that some birth defects are hereditary, passed from parents to offspring; little can now be done to prevent or cure these conditions, but genetic therapy offers hope that this situation may change in the future. Other birth defects result from infections of the mother during pregnancy, or from maternal consumption of alcohol or drugs, use of tobacco, or exposure to radiation or chemicals during pregnancy. In some cases, these birth defects can be prevented through education or improved prenatal care.

See also Embryo and embryonic development; Genetics.

Resources

books

Nussbaum, R.L., Roderick R. McInnes, Huntington F. Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.

Rimoin, D.L. Emery and Rimoin's Principles and Practice ofMedical Genetics. London; New York: Churchill Livingstone, 2002.

Sadler, T.W. and Jan Langman. Langman's Medical Embryology,, 8th ed. Lippincott Williams & Wilkins Publishers, 2000.

Larry Blaser

KEY TERMS

Hemoglobin

—The substance within red blood cells that gives blood its characteristic red color and carries oxygen to the cells of the body.

In utero

—While in the uterus, prior to birth.

Placenta

—The flat, plate-like organ of exchange between the blood of the mother and that of the embryo. It attaches to the wall of the uterus and provides nutrients and oxygen for the embryo and removes wastes from the embryo.

Birth Defects

Definition

Birth defects are physical abnormalities that are present at birth; they also are called congenital abnormalities. More than 3,000 have been identified.

Description

Birth defects are found in 2-3% of all newborn infants. This rate doubles in the first year, and reaches 10% by age five, as more defects become evident and can be diagnosed. Almost 20% of deaths in newborns are caused by birth defects.

Abnormalities can occur in any major organ or part of the body. Major defects are structural abnormalities that affect the way a person looks and require medical and/or surgical treatment. Minor defects are abnormalities that do not cause serious health or social problems. When multiple birth defects occur together and have a similar cause, they are called syndromes. If two or more defects tend to appear together but do not share the same cause, they are called associations.

Causes and symptoms

The specific cause of many congenital abnormalities is unknown, but several factors associated with pregnancy and delivery can increase the risk of birth defects.

Teratogens

Any substance that can cause abnormal development of the egg in the mother's womb is called a teratogen. In the first two months after conception, the developing organism is called an embryo; developmental stages from two months to birth are called fetal. Growth is rapid, and each body organ has a critical period in which it is especially sensitive to outside influences. About 7% of all congenital defects are caused by exposure to teratogens.

DRUGS. Only a few drugs are known to cause birth defects, but all have the potential to cause harm. For example, in 2003, a study found that use of topical (local) corticosteroids in the first trimester of pregnancy may be associated with cleft lip. Thalidomide is known to cause defects of the arms and legs; several other types also cause problems.

• Alcohol. Drinking large amounts of alcohol while pregnant causes a cluster of defects called fetal alcohol syndrome, which includes mental retardation, heart problems, and growth deficiency. In 2004, experts warned that binge drinking early in pregnancy was dangerous even if the woman quit drinking later.
• Antibiotics. Certain antibiotics are known tetratogens. Tetracycline affects bone growth and discolors the teeth. Drugs used to treat tuberculosis can lead to hearing problems and damage to a nerve in the head (cranial damage).
• Anticonvulsants. Drugs given to prevent seizures can cause serious problems in the developing fetus, including mental retardation and slow growth. Studies in the United Kingdom and Australia have tracked the percentage of birth defects caused by certain antiepileptic drugs.
• Antipsychotic and antianxiety agents. Several drugs given for anxiety and mental illness are known to cause specific defects.
• Antineoplastic agents. Drugs given to treat cancer can cause major congenital malformations, especially central nervous system defects. They also may be harmful to the health care worker who is giving them while pregnant.
• Hormones. Male hormones may cause masculinization of a female fetus. A synthetic estrogen (DES) given in the 1940s and 1950s caused an increased risk of cancer in the adult female children of the mothers who received the drug.
• Recreational drugs. Drugs such as LSD have been associated with arm and leg abnormalities and central nervous system problems in infants. Crack cocaine also has been associated with birth defects. Since drug abusers tend to use many drugs and have poor nutrition and prenatal care, it is hard to determine the effects of individual drugs.

CHEMICALS. Environmental chemicals such as fungicides, food additives, and pollutants are suspected of causing birth defects, though this is difficult to prove.

RADIATION. Exposure of the mother to high levels of radiation can cause small skull size (microcephaly), blindness, spina bifida, and cleft palate. How severe the defect is depends on the duration and timing of the exposure.

INFECTIONS. Three viruses are known to harm a developing baby: rubella, cytomegalovirus (CMV), and herpes simplex. Toxoplasma gondii, a parasite that can be contracted from undercooked meat, from dirt, or from handling the feces of infected cats, causes serious problems. Untreated syphilis in the mother also is harmful.

Genetic factors

A gene is a tiny, invisible unit containing information (DNA) that guides how the body forms and functions. Each individual inherits tens of thousands of genes from each parent, arranged on 46 chromosomes. Genes control all aspects of the body, how it works, and all its unique characteristics, including eye color and body size. Genes are influenced by chemicals and radiation, but sometimes changes in the genes are unexplained accidents. Each child gets half of its genes from each parent. In each pair of genes one will take precedence (dominant) over the other (recessive) in determining each trait, or characteristic. Birth defects caused by dominant inheritance include a form of dwarfism called achondroplasia; high cholesterol ; Huntington's disease, a progressive nervous system disorder; Marfan syndrome, which affects connective tissue; some forms of glaucoma, and polydactyly (extra fingers or toes).

KEY TERMS

Chromosome— One of the bodies in the cell nucleus that carries genes. There are normally 46 chromosomes in humans.

Cleft lip and palate— An opening in the lip, the roof of the mouth (hard palate), or the soft tissue in the back of the mouth (soft palate).

Embryo— The developing baby from conception to the end of the second month.

Gene— The The functional unit of heredity that directs all growth and development of an organism. Each human being has more than 100,000 genes that determine hair color, body build, and all other traits.

Fetus— In humans, the developing organism from the end of the eighth week to the moment of birth.

Neural tube defects— A group of birth defects that affect the backbone and sometimes the spinal chord.

Rubella— A mild, highly contagious childhood illness caused by a virus; it is also called German measles. It causes severe birth defects if a pregnant woman is not immune and gets the illness in the first three months of pregnancy.

Spina bifida— One of the more common birth defects in which the backbone never closes.

Trait— A distinguishing feature of an individual.

Virus— A very small organism that causes infection and needs a living cell to reproduce.

If both parents carry the same recessive gene, they have a one-in-four chance that the child will inherit the disease. Recessive diseases are severe and may lead to an early death. They include sickle cell anemia, a blood disorder that affects blacks, and Tay-Sachs disease, which causes mental retardation in people of eastern European Jewish heritage. Two recessive disorders that affect mostly whites are: cystic fibrosis, a lung and digestive disorder, and phenylketonuria (PKU), a metabolic disorder. If only one parent passes along the genes for the disorder, the normal gene received from the other parent will prevent the disease, but the child will be a carrier. Having the gene is not harmful to the carrier, but there is the 25% chance of the genetic disease showing up in the child of two carriers.

Some disorders are linked to the sex-determining chromosomes passed along by parents. Hemophilia, a condition that prevents blood from clotting, and Duchenne muscular dystrophy, which causes muscle weakness, are carried on the X chromosome. Genetic defects also can take place when the egg or sperm are forming if the mother or father passes along some faulty gene material. This is more common in older mothers. The most common defect of this kind is Down syndrome, a pattern of mental retardation and physical abnormalities, often including heart defects, caused by inheriting three copies of a chromosome rather than the normal pair.

A less understood cause of birth defects results from the interaction of genes from one or both parents plus environmental influences. These defects are thought to include:

• Cleft lip and palate, which are malformations of the mouth.
• Clubfoot, ankle or foot deformities.
• Spina bifida, an open spine caused when the tube that forms the brain and spinal chord does not close properly.
• Water on the brain (hydrocephalus ), which causes brain damage.
• Diabetes mellitus, an abnormality in sugar metabolism that appears later in life.
• Heart defects.
• Some forms of cancer.

A serious illness in the mother, such as an under-active thyroid, or diabetes mellitus, in which her body cannot process sugar, also can cause birth defects in the child. In fact, in 2003, it was shown that babies of diabetic mothers are five times as likely to have structural heart defects as other babies. An abnormal amount of amniotic fluid may indicate or cause birth defects. Amniotic fluid is the liquid that surrounds and protects the unborn child in the uterus. Too little of this fluid can interfere with lung or limb development. Too much amniotic fluid can accumulate if the fetus has a disorder that interferes with swallowing. In 2003, a study linked the mother's weight to risk of birth defects. Obese women were about three times more likely to have an infant with spina bifida or omphalocele (protrusion of part of the intestine through the abdominal wall) than women of average weight. Women who were overweight or classified as obese also were twice as likely to have an infant with a heart defect or multiple birth defects than women classified as average weight.

Diagnosis

If there is a family history of birth defects or if the mother is over 35 years old, then screening tests can be done during pregnancy to gain information about the health of the baby.

• Alpha-fetoprotein test. This is a simple blood test that measures the level of a substance called alpha-fetoprotein that is associated with some major birth defects. An abnormally high or low level may indicate the need for further testing.
• Ultrasound. The use of sound waves to examine the shape, function, and age of the fetus is a common procedure. It also can detect many malformations, such as spina bifida, limb defects, and heart and kidney problems. In 2003, researchers in England announced a new combination of blood tests and ultrasound to detect Down syndrome sooner and more accurately than with the usual blood screenings done at 20 weeks of pregnancy.
• Amniocentesis. This test usually is done between the 13th and 15th weeks of pregnancy. A small sample of amniotic fluid is withdrawn through a thin needle inserted into the mother's abdomen. Chromosomal analysis can rule out Down syndrome and other genetic conditions.
• Chorionic villus sampling (CVS). This test can be done as early as the ninth week of pregnancy to identify chromosome disorders and some genetic conditions. A thin needle is inserted through the abdomen or a slim tube is inserted through the vagina that takes a tiny tissue sample for testing.

If a birth defect is suspected after a baby is born, then confirmation of the diagnosis is very important. The patient's medical records and medical history may hold essential information. A careful physical examination and laboratory tests should be done. Special diagnostic tests also can provide genetic information in some cases. In 2003, the March of Dimes, a nonprofit organization, recommended that every baby born in the United States receive, at minimum, screening for the same core group of birth defects including phenylketonuria, congenital adrenal hyperplasia, congenital hypothryroidism, biotinidase deficiency, and others. They were concerned that newborn screening varied too much from state to state.

Treatment

Treatment depends on the type of birth defect and how serious it is. When an abnormality has been identified before birth, delivery can be planned at a health care facility that is prepared to offer any special care needed. Some abnormalities can be corrected with surgery. Experimental procedures have been used successfully in correcting some defects, like excessive fluid in the brain (hydrocephalus), even before the baby is born. Early reports have shown success with fetal surgery on spina bifida patients. By operating on these fetuses while still in the womb, surgeons have prevented the need for shunts and improved outcomes at birth for many newborns. However, long-term studies still are needed. Patients with complicated conditions usually need the help of experienced medical and educational specialists with an understanding of the disorder.

Prognosis

The prognosis for a disorder varies with the specific condition.

Prevention

Pregnant women should eat a nutritious diet. Taking folic acid supplements before and during pregnancy reduces the risk of having a baby with serious problems of the brain or spinal chord (neural tube defects). It is important to avoid any teratogen that can harm the developing baby, including alcohol and drugs. When there is a family history of congenital defects in either parent, genetic counseling and testing can help parents plan for future children. Often, counselors can determine the risk of a genetic condition occurring and the availability of tests for it. Talking to a genetic counselor after a child is born with a defect can provide parents with information about medical management and available community resources.

Resources

PERIODICALS

"Babies of Diabetic Mothers Have Fivefold Increase in Structural Heart Defects." Diabetes Week (October 6, 2003): 8.

Bauer, Jeff. "Researchers Link Momós Weight to Babyós Risk of Birth Defects." RN (August 2003): 97-102.

"Fetal Alcohol Syndrome Is Still a Threat, Says Publication." Science Letter (September 28, 2004): 448.

"Fetal Diagnostic Test Combo Shows Promise." Health & Medicine Week (October 27, 2003): 224.

"Fetal Surgery for Spina Bifida Shows Benefits in Leg Function, Fewer Shunts." Health & Medicine Week (October 20, 2003): 608.

"March of Dimes Pushes Newborn Screening." Diagnostics & Imaging Week (July 31, 2003): 10-11.

"Studies Reveal Risk of Birth Defects from AEDs." Pharma Marketletter (September 13, 2004).

"Topical Corticosteroids Use During Pregnancy May Associate With Cleft Lip." Biotech Week (September 24, 2003): 190.

ORGANIZATIONS

March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. (914) 428-7100. resourcecenter@modimes.org. 〈http://www.modimes.org〉.

OTHER

March of Dimes. Public Health Education Information Sheets.

Birth Defects

How Do Hereditary Factors Cause Birth Defects?

How Do Environmental Factors Cause Birth Defects?

How Do Doctors Diagnose Birth Defects?

How Do Doctors Treat Birth Defects?

How Do Doctors and Parents-to-Be Prevent Birth Defects?

Resources

A birth defect is an abnormality in the body structure or chemistry of a newborn child. It may be caused by hereditary factors (that is, by genetic causes), by environmental influences that affect the embryo or fetus in the mother’s womb, or by a combination of factors. Often, the cause of a birth defect is unknown.

KEYWORDS

for searching the Internet and other reference sources

Genetics

Pregnancy

Amniocentesis

Infant mortality

Birth defects sometimes are called congenital anomalies (kon-JEN-i-tal a-NAM-a-leez). Congenital means present at birth, and anomalies are abnormalities or irregularities. An abnormality present at birth usually is not considered a birth defect unless it results in a disease or in a physical or mental disability. For example, birthmarks are seldom considered birth defects because they usually do not cause health problems.

The March of Dimes estimates that 3 to 5 percent of babies born in the United States have some type of birth defect. Some birth defects, such as cleft palate, occur infrequently. Others, such as some congenital heart defects, are more common. Some hereditary defects are more common in certain populations than in others. For example, sickle-cell anemia, an inherited blood disease, occurs mainly in people of African ancestry, whereas Tay-Sachs disease, a fatal disorder of body chemistry, primarily affects people of eastern European Jewish ancestry.

How Do Hereditary Factors Cause Birth Defects?

Each of us has genes* that are inherited from our parents. The genes occur in pairs along threadlike bodies called chromosomes*, which are located in the nucleus* of each cell in the body. Genes determine our inborn characteristics, or traits. These traits include how we look and how the chemical substances in our bodies function. In the case of birth defects, the genes also may determine abnormalities if they are faulty in some way.

* genes

are chemicals in the body that help determine a person’s characteristics, such as hair or eye color. They are inherited from a person’s parents and are contained in the chromosomes found in the cells of the body.

* chromosomes

are thread-like structures inside cells on which the genes are located.

* nucleus

is the part of the cell that contains its genetic information.

Mendelian inheritance patterns

The simplest patterns of birth defect inheritance are called Mendelian, named for the Austrian monk Gregor Mendel who observed them in the nineteenth century. In Mendelian inheritance, traits (including defects) can be transmitted by way of dominant or recessive genes.

It works this way: A child inherits two copies of each gene, one from the mother and one from the father. If a defective gene is dominant, a child who inherits even one copy of it will have the defect. That is because the defective copy “dominates,” or overwhelms, the normal copy inherited from the other parent. But if a defective gene is recessive, the child would have to inherit two defective copies—one from the mother and one from the father—in order to have the defect. A person who inherited only one defective copy would be healthy but could pass the defective copy on to his or her own children. These kinds of inheritance patterns cause what researchers call autosomal (aw-to-SO-mal) birth defects.

Examples of autosomal dominant birth defects are Huntington’s disease, a nervous system disorder, and Marfan syndrome, which is characterized by tallness, elongated bones, and heart problems. Some birth defects, such as Huntingtons disease, may not show symptoms for many years.

Other birth defects are determined by genes located on the X chromosome (the X and Y chromosomes determine the sex of an infant). Such abnormalities are said to be X-linked. Hemophilia, a blood disorder, and color blindness are examples of X-linked birth defects.

Many hereditary birth defects, however, are not simply dominant, recessive, or X-linked. They may be produced instead by multiple faulty genes.

Chromosome abnormalities

Some birth defects are caused by extra, missing, incomplete, or misshapen chromosomes. Down syndrome, one of the most common birth defects, usually is caused by the presence of an extra chromosome in the cells. Down syndrome produces mental retardation, short stature, and distinctive facial features. Defects involving the sex chromosomes can produce problems in sexual development, including sterility, which is an inability to have children.

The U.S. and the World

• An estimated 515,000 people worldwide died of birth defects in 1998. Almost 93 percent of those deaths were in low- and middle-income nations.
• More than 28 million people worldwide live with birth defects, and about 93 percent live in countries with low or middle income.
• Birth defects are the leading cause of infant mortality in the United States. In 1997,6,178 children died before their first birthdays because they had one or more birth defects. That number is 22 percent of all children who died underage 1.
• The most common birth defects involve the heart. About 28 percent of the children under age 1 who died of birth defects in 1997 had problems with their hearts. The second most common category comprised defects of the respiratory system, accounting for about 16 percent of the deaths that year.
• The death rate from birth defects in the United States dropped 37.7 percent between 1979 and 1997. One reason for the decrease is improved medical care for infants with birth defects.
• Children with birth defects who survive their first year still may die of complications from the defects later in life. In 1997,11,912 people of all ages died of birth defects, with only about half under age 1.
• About 150,000 babies a year are born with a birth defect in the United States. The rate in the mid-1990s put the United States twenty-fifth in the world, with twice as many birth defects per 1,000 live births as the nation with the lowest rate, Japan.

How Do Environmental Factors Cause Birth Defects?

Birth defects also can be caused by environmental factors, either alone or together with faulty genes. “Environmental” here refers to the environment in the mother’s uterus, or womb, rather than to the earth’s environment. However, scientists are studying the possible influence on birth defects of poisons in the earth’s environment.

Pregnant women who consume excessive amounts of alcohol during the early stages of pregnancy risk having babies with fetal alcohol syndrome. Children with this disorder may have various defects in growth, facial appearance, and mental ability. Scientists are studying whether even moderate use of alcohol can damage a fetus. Smoking during pregnancy increases the likelihood that the baby will have lower than normal weight at birth, increasing the risk of defects.

Some illnesses in a pregnant woman can cause damage to the fetus. For example, German measles (rubella) can cause deafness, blindness, and heart defects in the newborn. Sexually transmitted diseases also can be transmitted to the fetus or to the newborn at birth.

Certain medications have been linked to birth defects. The most famous is the drug thalidomide (tha-LID-o-mide), a sedative that in the early 1960s was found to cause greatly shortened arms and legs in many newborn infants. Many other medications, including tranquilizers and antibacterial and anticancer drugs, can cause congenital abnormalities.

Other environmental factors believed to increase the risk of birth defects include poor nutrition and the age of the mother. For example, the older a pregnant woman is, the more likely she is to give birth to a child with Down syndrome. If the expectant mother is 35 or older, experts recommend that the fetus be tested.

How Do Doctors Diagnose Birth Defects?

Some birth defects can be diagnosed while the child is still in its mother’s womb. A procedure called ultrasound, which uses sound waves to produce an image of a fetus on a screen, can detect some malformations. For example, a defect called spina bifida, in which a part of the spinal cord is exposed, can be discovered by ultrasound.

In a procedure called amniocentesis (am-nee-o-sen-TEE-sis), a small sample of fluid surrounding the fetus is removed through a needle and examined. This test is useful in detecting inborn metabolic (body chemistry) defects and abnormalities in the chromosomes.

Many birth defects can be diagnosed by a doctor’s physical examination of a newborn baby. Other tests, including x-rays, may be ordered if doctors suspect a birth defect; blood tests can detect certain disorders of the blood or body chemistry. Many infants with defects can develop normally if they receive prompt treatment.

Thalidomide and Phocomelia

Thalidomide is a sedative, a medication prescribed to calm the nerves. If taken in the early months of pregnancy it can cause a birth defect called phocomelia (fo-ko-MEE-lee-a), in which the arms, legs, or both are very short and underdeveloped.

Thalidomide was prescribed for pregnant women in Europe in the late 1950s and early 1960s until it was linked to phocomelia in thousands of newborn babies. Fortunately for Americans, thalidomide never was sold in the United States because the U.S. Food and Drug Administration refused to approve it. An employee of this federal agency, Dr. Frances Kelsey, insisted that thalidomide’s safety had not been proven. Many people think Dr. Kelsey saved hundreds or even thousands of children from severe birth defects.

The thalidomide tragedy led to an increase in government control over the marketing of drugs throughout the world, and the use of thalidomide is now banned or strictly controlled in most countries.

Thalidomide may prove helpful in the treatment of such serious diseases as AIDS and leprosy, and researchers are continuing to study its therapeutic uses.

How Do Doctors Treat Birth Defects?

Not every birth defect affects the quality of life of the person who has it. Some birth defects have little effect, except perhaps on appearance.

Several birth defects can be treated to prevent or reduce their harmful effects. Surgeons can perform operations to correct such malformations as clubfoot, cleft palate, cleft lip, and structural defects in the heart and digestive tract. Treatment can lessen the symptoms of cystic fibrosis, an inherited disease that interferes with breathing. In some cases, disorders such as hydrocephalus (hy-dro-SEF-a-lus), a damaging buildup of fluid in the head, can be remedied even before birth.

Doctors can sometimes treat inborn disorders of body chemistry with medications and special diets. For example, prompt treatment can prevent brain damage in phenylketonuria (fen-il-kee-to-NOOR-ee-a), or PKU, a metabolic defect that can produce severe mental retardation. Special education, rehabilitation, and the use of special devices and machines can help to offset some mental and physical handicaps, such as blindness and deafness.

How Do Doctors and Parents-to-Be Prevent Birth Defects?

No one can guarantee that a baby will be born “perfect” and healthy. However, there are ways to minimize the likelihood of having a child with a preventable birth defect. Some of the most important ways involve lifestyle. A prospective mother should understand that many things she does may have an effect on the new life growing inside her:

• Pregnant women should not smoke or drink alcoholic beverages, nor should they use drugs of any kind unless prescribed by a doctor.
• Certain vitamins, if taken in proper amounts by the mother-to-be, can help prevent some birth defects. For example, folate (folic acid) taken during pregnancy can help prevent certain defects of the spinal column and central nervous system, including spina bifida.
• Vaccination well before pregnancy can prevent birth defects that might occur if the mother were to develop German measles while pregnant.

Couples should seek genetic counseling before planning to have a child if the mother, father, or relatives have hereditary abnormalities.

See also

Cleft Palate

Color Blindness

Cystic Fibrosis

Deafness and Hearing Loss

Down Syndrome

Fetal Alcohol Syndrome

Genetic Diseases

German Measles (Rubella)

Hemophilia

Huntington’s Disease

Hydrocephalus

Marfan Syndrome

Sexually Transmitted Diseases

Spina Bifida

Tay-Sachs Disease

Resources

Books

Kidd, J. S., and R. A. Kidd. Life Lines: The Story of the New Genetics. New York: Facts on File, 1999. This survey for young adults discusses the evolution of the study of genetics.

Marshall, Elizabeth L. The Human Genome Project: Cracking the Code Within Us. New York: Franklin Watts, 1997. A look at this international project and the scientists doing the research.

Organizations

U.S. Centers for Disease Control and Prevention (CDC). CDC posts several fact sheets about birth defects at its website. http://www.cdc.gov/health/diseases.htm http://www.cdc.gov/nceh/programs/infants/brthdfct/prevent/bd_rev.htm

The March of Dimes Birth Defects Foundation, 1275 Mamaroneck Avenue, White Plains, NY 10605. The mission of this national organization is to improve the health of babies by preventing birth defects and infant mortality. Its website offers information on the cause and prevention of birth defects. Telephone 888-663-4637 http://www.modimes.org