Causes of Birth Defects

Causes of Birth Defects
Although the causes of most birth defects are un-
known, many are attributable to a combination of fac-
tors. Some birth defects are the result of genetic
determinants, such as an abnormality due to an in-
herited trait or a problem with a gene or chromo-
some. For instance, researchers have linked various
physical malformations, metabolic abnormalities, cer-
tain 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 num-
ber. Down syndrome, which may lead to mental retar-
BIRTH DEFECTS  55dation, 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