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 syn-
drome affects 1 in 900 births, and there is a substan-
tially 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 abnor-
malities. 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 physi-
cal and mental abnormalities.
Several birth defects can be traced to a mutation
in a single gene or chromosome (e.g., neurofibroma-
tosis type 1 and cystic fibrosis) or environmental influ-
ence (e.g., thalidomide, rubella virus, and ionizing
irradiation), but most are due to a combination of
these factors. This is referred to as multifactorial in-
heritance. 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 condi-
tions 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 environmen-
tal factors such as maternal alcohol consumption,
which has been observed at higher rates among Na-
tive 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 de-
fect, 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 severi-
ty 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 de-
fects or ventricular enlargement, may be a result of
using alcohol or certain medications during pregnan-
cy. Mutations in certain genes have also been report-
ed to cause some of the defects. Some malformations
can be repaired with surgery. Although these types of
birth defects are not completely preventable, a preg-
nant woman can reduce risk by discussing medica-
tions 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 an-
encephaly, spina bifida, and encephalocele, are seri-
ous 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 Admin-
istration issued a rule (effective January 1, 1998) re-
quiring 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 infec-
tions, prevention initiatives also emphasize immuni-
zation 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 preg-
nant 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 prevent-
able, prevention is complicated by the fact that most
serious birth defects occur during the early weeks of56 BIRTH DEFECTS