Birth Defects

I

Introduction

Birth Defects, also called congenital disorders, any abnormalities of structure or function that are present at birth. The process of fetal development can be disrupted by a variety of external factors such as exposure to radiation, heat, chemicals, infectious agents, or maternal disease. An external agent that alters fetal development is called a teratogen (Greek teratos, “monster”; genes, “born”). Developmental malformations can also be the result of abnormalities in the genetic makeup of the fetus, or they can be caused by a combination of genetic and environmental influences.

Twenty percent or more of malformed fetuses are spontaneously aborted; the rest result in a newborn with a birth defect. Although each single type of birth defect is rare, taken together they make up almost 5 percent of all live births and cause about 20 percent of infant deaths in the period immediately after birth. About one in ten developmental disorders is hereditary and arises from an abnormality in a single gene. Another 5 percent of birth defects arise from physical abnormalities in the chromosome.

II

Genetic Causes

Some genetic disorders have symptoms that are manifest at birth. These disorders may result from mutations in a single gene or from more general chromosomal abnormalities. Many diseases and conditions are inherited in a recessive manner: Neither parent may have the defect even though they both carry the causative gene. When both parents have a dominant gene A and a recessive gene a, their offspring may inherit one of four different combinations: AA, Aa, aA, or aa. If the recessive gene a is defective, the statistical probability is that one in four of the offspring will bear the defective trait. In other congenital disorders the presence of only one copy of the recessive gene is sufficient to cause the condition. See also Genetics.

III

External Causes

Approximately one in ten birth defects is the result of some force or factor that comes from outside the human body. For example, the effects of radiation were demonstrated by the increased incidence of birth defects in the offspring of pregnant Japanese women who were exposed to the atomic bomb in 1945 and of American women who underwent radiation therapy while pregnant. Although the risk from a diagnostic X ray is slight, radiologists recommend having only those X rays immediately necessary during pregnancy. Elevated temperature in a pregnant woman (such as that experienced by sitting in a hot tub) can also lead to birth defects.

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The belief that the placenta, which unites the fetus to the maternal uterus, is a protective barrier against chemicals in the mother’s blood was tragically disproved in the late 1950s and early 1960s. In many countries children were born with limbs missing or arrested in development after their mothers took the sedative thalidomide, which had not been approved for distribution in the United States. Other known teratogens include alcohol, anticonvulsants, chemotherapeutic agents, cocaine, retinoic acid (a treatment for acne), and the antibiotics streptomycin and tetracycline. In 1955 an outbreak of congenital cerebral palsy in Japan was found to be caused by pregnant women’s consumption of fish contaminated with the industrial chemical methylmercury.

A number of infections, when contracted by a pregnant woman, can endanger her unborn child. One of these is rubella, or German measles, which can lead to mental retardation and abnormalities in sight and hearing in the newborn. Vaccination of girls during childhood or adolescence can prevent a later infection during pregnancy. Other maternal infections occurring during pregnancy that can damage a fetus include acquired immunodeficiency syndrome (AIDS), chicken pox, toxoplasmosis, and cytomegalovirus.

Women with insulin-dependent diabetes mellitus who are not adequately treated during pregnancy are subject to an increased risk of giving birth to a child with heart defects and a variety of other problems. Poorly controlled phenylketonuria (see Metabolism) in a pregnant woman can also lead to multiple birth defects and mental retardation in the child.

IV

Polygenic and Multifactorial Causes

No definite cause is known as yet for about two-thirds of birth defects. Some spine and heart malformations are thought to be polygenic, that is, the result of several abnormal genes that are present simultaneously. Other appear to be multifactorial, resulting from abnormal genes interacting with harmful environmental factors. Some birth defects are found more frequently in children with older parents. It has been found that the risk of Down Syndrome, for instance, rises with the increasing age of the mother.

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