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 retar dation, 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.
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