This article is from the General Pregnancy FAQ, by swnymph@FensEnde.com (Sabrina Cuddy) with numerous contributions by others.
In earlier days, doctors believed that the placenta acted as a
"barrier" to dangerous agents, protecting the fetus from anything its
mother was exposed to. Now we know that most drugs easily cross the
placenta and reach the fetus.
Drugs or other agents that pass through the placenta to affect the
fetal environment are called environmental factors. And environmental
factors that have been lined definitively to birth defects are called
teratogens. Besides drugs, other known teratogens are heavy metals
(like lead) and certain chemicals to which a pregnant woman may
inadvertently expose herself ad her unborn baby.
Most fetal organ formation occurs during the first three months of
pregnancy, thus making the first trimester the most dangerous time for
exposing an unborn baby to teratogens. The teratogenic period -- the
time when birth defects caused by environmental factors are most likely
to occur -- is the period from two to eight weeks of fetal development.
(If an embryo is exposed to a teratogen in the first two weeks of the
first trimester, it usually causes an "all or none" effect -- meaning
the pregnancy will end in miscarriage or the fetus will be unaffected.)
But fetal development continues beyond the first trimester, and the
developing baby remains vulnerable. The female genital system, for
instance, does not completely form until well into the fifth month, and
the brain continues its development throughout pregnancy. Therefore,
damage tot he fetus's biochemistry and physiology may occur in the
second half of pregnancy. For example, the antibiotic tetracycline may
impair bone development when exposure occurs after the 20th week of
pregnancy. And the drug Coumadin, which acts as a blood thinner in the
adult, may cause internal bleeding in the fetus in the last half of
pregnancy -- and can linger to cause this in the baby after birth.
Likewise, tranquilizers and other drugs affecting the central nervous
system may cause lethargy or decreased sucking in a newborn, or may
have a more delayed impact in the form of subtle behavioral changes
that appear later on in childhood.
The risk to a fetus from exposure to drugs is especially high during
the period between conception and the time a woman realizes she's
pregnant. During those weeks, a woman may take over-the-counter and
even recreational drugs without knowing she's putting her baby in
Many pregnant women may think of over-the-counter drugs as harmless
because these medications are available with a prescription. Even
after pregnancy is diagnosed, most western countries tend to support
the concept of using these medications to alleviate pregnancy
discomforts and symptoms as a relatively harmless practice. However,
some over-the-counter drugs can cause problems for an unborn baby.
(For a list of over--the-counter drugs that are generally considered
safe for pregnant women, see attachment at the end of article.)