Environmental Causes

Numerous cases of mental retardation are a result of damage to a fetus during
pregnancy. Other problems may arise during birth or after birth. Physical or chemical agents that cause an increase in congenital defects are
known as teratogens. Because teratogens affect embryos and fetuses directly,
the effects are not likely to produce heritable changes. A woman who
uses or is exposed to various teratogens during pregnancy runs the risk of
producing a child with a developmental malformation. Potential teratogens
include alcohol, drugs, viral infections, radiation, diabetes mellitus, malnutrition,
and environmental toxins.
Since its initial clinical delineation in 1973, fetal alcohol syndrome has
been noted as a major cause of mental retardation in countries where alcohol
is consumed regularly. Estimates indicate that it may be responsible for
as many as one to three cases of mental retardation out of every thousand
births. Fortunately, fetal alcohol syndrome is easily preventable through abstinence
from alcohol during pregnancy. Children affected with fetal alcohol
syndrome have a characteristic facial appearance, with a small skull, upturned
nose, thin upper lip, underdeveloped upper jaw, epicanthal folds,
and a long philtrum (the vertical groove on the median line of the upper lip).
There is growth retardation, which has its onset prenatally and continues
during the postnatal period with some catch-up growth taking place thereafter.
Head and brain size remain well below normal. Children show developmental
delays, attention deficits, hyperactivity, and mental deficiency. Although
the average IQ of children with fetal alcohol syndrome is low, 60 to
65, there is considerable variation, with some children having normal or nearnormal
intelligence but experiencing learning disorders. Severe physical
defects found in many of these children include cardiac and skeletal defects.
Although it is evident that the risk of fetal alcohol syndrome is related to
the amount and timing of the alcohol consumed by the pregnant woman,
an exact close relationship has been difficult to establish. Even with moderate
consumption (one to two ounces of absolute alcohol), the serious effects
of fetal alcohol syndrome have been observed in approximately 10 percent
of births. Many physicians now recommend that women practice total abstinence
from alcohol during the entire pregnancy. 537