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.