BIRTH DEFECTS

Causes of Birth Defects

Although the causes of most birth defects are unknown, many are attributable to a combination of factors. Some birth defects are the result of genetic determinants, such as an abnormality due to an inherited trait or a problem with a gene or chromosome. For instance, researchers have linked various physical malformations, metabolic abnormalities, certain vision and hearing losses, and other birth defects to specific genes that are inherited from one (or in rare cases, both) parent. Problems may also arise from defects in a gene or chromosome structure or number. Down syndrome, which may lead to mental retardation, cardiac difficulties, and other problems, is caused by an extra copy of chromosome 21. As one of the most common serious birth defects, Down syndrome affects 1 in 900 births, and there is a substantially increased risk of giving birth to a child with Down syndrome if the mother is over thirty-five years of age.

Myriad environmental, or nongenetic, factors have also been linked to birth defects. Prescription and nonprescription medications, illicit drugs, and other harmful chemicals can cause newborn abnormalities. Alcohol use during pregnancy has been linked to fetal alcohol syndrome, which occurs about once in every 1,000 births. Infants with fetal alcohol syndrome are born with a range of preventable physical and mental abnormalities.

Several birth defects can be traced to a mutation in a single gene or chromosome (e.g., neurofibromatosis type 1 and cystic fibrosis) or environmental influence (e.g., thalidomide, rubella virus, and ionizing irradiation), but most are due to a combination of these factors. This is referred to as multifactorial inheritance. Neural tube defects and orofacial clefts (cleft lip and cleft palate) are two types of anomalies that are thought to have a multifactorial cause in most instances. Cleft lip, which results from an incomplete development of the lip, and cleft palate, which is an incomplete development of the roof of the mouth, may occur singly or in combination with each other. Cleft lip with or without cleft palate occurs more often than cleft palate alone, but infants with cleft palate alone are much more likely to have birth defects that involve other organ systems and are more likely to have chromosomal anomalies. Although these conditions can be remedied through surgery, speech and hearing difficulty may be associated with cleft palate. The complexity of the causes of these birth defects are apparent in that they are associated with environmental factors such as maternal alcohol consumption, which has been observed at higher rates among Native Americans and Caucasians and relatively low rates in African Americans, and that there is increased risk for infants born to a parent with a cleft lip and/or palate.

Heart defects, the most common type of birth defect, affect about 25,000 infants each year and are considered to have a multifactorial genesis. Because of improvements in diagnostic techniques such as echocardiography, the number of infants diagnosed with heart defects has increased dramatically in the 1980s and 1990s. Heart defects vary greatly in severity and can occur in isolation or can be one component of a complex syndrome (such as Down syndrome). Malformations of the heart, such as atrial septal defects or ventricular enlargement, may be a result of using alcohol or certain medications during pregnancy. Mutations in certain genes have also been reported to cause some of the defects. Some malformations can be repaired with surgery. Although these types of birth defects are not completely preventable, a pregnant woman can reduce risk by discussing medications she is using with her doctor and by avoiding alcohol.

Prevention of Birth Defects

In the past ten years, there have been significant strides in understanding ways to prevent some birth defects. For example, a daily supplement to the diet of 500 micrograms of folic acid, a B vitamin, has been shown to prevent up to 70 percent of cases of neural tube defects. Neural tube defects, which include anencephaly, spina bifida, and encephalocele, are serious and often lethal birth defects of the spine and central nervous system. The recognition that many of these birth defects can be prevented with folic acid has led to initiatives at the state and national levels aimed at educating women about the importance of consuming the appropriate amount of this vitamin on a daily basis. In 1996 the U.S. Food and Drug Administration issued a rule (effective January 1, 1998) requiring that all enriched grain products sold in the United States be fortified with 140 micrograms of folic acid per 100 grams of product. As a result of these public health initiatives, the rate of spina bifida and anencephaly has declined substantially since the early 1990s.

Because several birth defects are caused by infections, prevention initiatives also emphasize immunization and information. For example, because of widespread vaccination for rubella (German measles), the birth defects caused by this infection rarely occur in the United States. Information about the risk of birth defects resulting from maternal infection with syphilis or other sexually transmitted diseases may stimulate the development of services to help women at greatest risk. Cytomegalovirus, the most common of the congenital viral infections, affects almost 40,000 infants each year. It can be passed through bodily fluids, such as saliva, blood, and breast milk. It is often passed to a pregnant woman from a child who is infected but is not showing symptoms; for example, an infected child may sneeze and then touch a pregnant woman, thus infecting her. An infant born to a mother who has contracted cytomegalovirus is at an increased risk for mental retardation and vision or hearing loss.

Although many types of birth defects are preventable, prevention is complicated by the fact that most serious birth defects occur during the early weeks of pregnancy, often before a woman even knows she is pregnant. This is why strategies aimed at preventing birth defects must focus on improving the health of women prior to pregnancy. Screening and diagnostic tests, such as ultrasound, maternal serum a-fetoprotein screening, amniocentesis, and chorionic villus sampling, are used to monitor the health of the fetus and to identify certain fetal malformations and chromosomal disorders; they cannot, however, be used to prevent these conditions from occurring. Decisions about whether to use prenatal testing, which tests are appropriate, and how to use the results must be made by the mother in conjunction with her physician.

Consequences of Birth Defects

An infant with a birth defect presents many challenges both for the child and the family. Children with sensory abnormalities, such as hearing or sight loss, have been shown to experience the greatest difficulty in psychosocial adjustment, whereas children with cardiac malformations experience maladjustment to a lesser extent. There have not been many studies addressing either the type of psychological problems or the long-term effects experienced by children with birth defects. A study of over 3,000 children in Canada reported that most children with cystic fibrosis (an inherited gene mutation that causes problems with the lungs, pancreas, and other organs) have some type of major psychiatric diagnosis, with anxiety disorder being the most common. Long-term research is needed, however, to assess any lasting effects of a child's condition on his or her mental and emotional well-being.

One component of a child's psychosocial development is related to social pressure. Studies have found that individuals with spina bifida and Down syndrome do not perceive themselves as sick. Many of the social difficulties experienced by children with birth defects are not caused directly by the anomaly but by the expectations of what is normal and expected in their communities.

Studies of the families of children with birth defects have focused on psychological stresses experienced by mothers. Mothers of infants with very low birthweights (which is a factor closely related to birth defects) experience greater psychological stress than mothers of normal weight infants. Overall, studies have shown that families of children with birth defects may experience more distress, as measured by higher levels of mental health treatment, than families of children without birth defects. These families, however, are no more prone to divorce, social isolation, or alcohol problems than families without a child affected by a birth defect.

There are a growing number of web-based resources for information about birth defects. The March of Dimes and the Centers for Disease Control and Prevention provide information and links to other web sites for information about specific conditions. Additionally, there are state and national birth defect monitoring programs. The purpose of these projects is to conduct surveillance about birth defects to target information dissemination, track changes in prevalence, and identify trends. This information stimulates research about prevention and affects program development. Several states use information from their birth defects registries to refer infants and their families to appropriate services.

See also: TERATOGENS

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Anita Farel

Robert Meyer

Maggie Hicken