The conventional wisdom: Expectant mothers should avoid all nonessential drugs, whether prescription or over-the-counter.

Our current understanding is as follows: First, antidepressants do not appear to cause major birth defects (with the exception of Paxil—the FDA recently linked it to heart malformations in babies). Second, there's not enough data to determine if antidepressants harm the fetal brain. Third, a mother's depression can definitely harm fetuses and newborns.

Depression during pregnancy is both common and potentially serious, occurring in at least 1 in 10 expectant women. It has been linked to miscarriage, premature birth, low birth weight, fetal growth problems, excessive crying and other signs of infant distress as well as preeclampsia, a type of hypertension which can be fatal to both mother and child.

Furthermore, it's known that women who are depressed before giving birth often experience postpartum depression. And among women who stop antidepressants before conception or in early pregnancy, some 75 percent will relapse into depression, according to a recent study in the Archives of Women's Mental Health.

To Gideon Koren, professor of pediatrics and pharmacology at the University of Toronto, the risks of untreated depression outweigh what he sees as largely speculative risks associated with drugs. One study by his group, published in the American Journal of Psychiatry, compared children of women who took antidepressants while pregnant to those whose mothers were not depressed. It found no differences in behavior, temperament or IQ associated with medication in children up to 6 years old.

Long periods of maternal depression after the birth, however, are linked with lower IQs in children. A mother's depression may also affect speech development in her kids. Young children whose mothers suffer repeated bouts of depression tend to score lower on language tests. The evidence suggests that better treatment for depression benefits both mother and child.

Other researchers are much less sanguine. Sandy Zeskind, a professor of pediatrics at the University of North Carolina, points to the fact that serotonin—a neurotransmitter affected by these drugs—plays a critical role in brain development, acting as a growth factor, not just a chemical messenger. It helps wire a baby's brain, telling neurons where to go.

At birth, up to 30 percent of infants born to women who take antidepressants suffer withdrawal or toxicity syndrome, in which they may be jittery, suffer breathing problems or even have seizures. The symptoms sound horrific, but research finds that they aren't long-lasting and have never produced a fatal outcome. Testing for the level of medication in the infant can determine whether symptoms are associated with withdrawal (low levels) or toxicity (high levels), and thus whether tapering doses should be given.

Studies have also found differences in motor skills and in pain sensitivity in babies exposed to antidepressants in the womb.

Is there another way? Because the third trimester is the most critical period for serotonin exposure, some physicians recommend that women taper off medication during the second trimester and restart it after birth if needed. Such an approach might best balance risks and benefits—especially since breast-feeding does not appear to expose most babies to high drug levels. Using the lowest effective dose—not the lowest possible dose—is another way to reduce risk.

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