B

birth defects More than 150,000 infants born
each year in the United States have structural or
functional abnormalities present at birth, ranging
from mild to severely debilitating or fatal. Sub-
stances and circumstances that can cause birth
defects are teratogenic. Some birth defects,
notably FETAL ALCOHOL SYNDROME(FAS) and those
that occur as a SIDE EFFECTof medications, are
entirely preventable. GENE mutations, some of
which are hereditary mutations and many of
which are spontaneous or isolated mutations,
cause many birth defects. The risk for the genetic
condition DOWN SYNDROME(trisomy 21), a chromo-
somal disorder, rises with the mother’s age, the
only birth defect doctors know for certain does so.
Many other birth defects are not preventable,
however, and may not be detectable before birth.
The risk for many other birth defects correlates
to exposures during PREGNANCY, such as to the viral
infections RUBELLA(German measles), CYTOMEGA-
LOVIRUS(CMV), and CHICKENPOX, and to the parasitic
INFECTION TOXOPLASMOSIS. These infections can
cause mild to significant birth defects, ranging
from congenital CATARACT and HEARING LOSS to
HEART malformations. Avoiding these exposures
prevents any consequential damage to the devel-
oping FETUS.

Teratogenic Medications
Doctors widely prescribed the DRUGthalidomide in
the 1950s and early 1960s as a treatment for
MORNING SICKNESSuntil they discovered the high
incidence of limb deformities associated with its
use. Thalidomide marked a turning point in public
awareness about the teratogenic hazards of med-
ications as well as in research efforts to identify
those hazards. Numerous medications can cause

birth defects. The US Food and Drug Administra-

tion (FDA), which regulates drug approval and

use in the United States, assigns pregnancy cate-

gories to medications to help doctors and women

assess the risks of using the medications during

pregnancy. These categories are

• pregnancy category A: medications for which
  numerous clinical studies have shown no
  adverse effects in pregnancy


• pregnancy category B: medications for which
  animal studies have shown no adverse effects
  or for which there are limited studies


• pregnancy category C: medications for which
  there are no studies to indicate either safety or
  hazard during pregnancy


• pregnancy category D: medications for which
  clinical studies demonstrate risk to the develop-
  ing fetus, although the benefits to the mother
  of the medication may outweigh the risks to
  the fetus


• pregnancy category X: medications for which
  clinical studies demonstrate clear evidence of
  damage to the developing fetus

Doctors generally consider pregnancy category

A and B medications safe for women to use during

pregnancy, though approach the use of pregnancy

category C medications with caution. Women who

are pregnant or planning pregnancy should thor-

oughly discuss benefits and risks with their doc-

tors before taking or continuing to take category D

medications and should never take pregnancy cat-

egory X medications. Pregnant women should

check with their doctors or pharmacists about any

medications they are taking when they become

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