When pregnant women take antidepressants, it sometimes causes their babies to hit developmental milestones late, Danish researchers reported on Monday.

However, the delays-up to one month-still place the toddlers within the normal range of development.

“These drugs have an effect on the fetus’ brain,” said Dr. Lars Henning Pedersen, who worked on the study. But, he said, the delays “may not matter for the child at all.” Pedersen, from Aarhus University Hospital in Denmark, spoke to Reuters Health in a telephone interview.

Today, as many as one in six pregnant women in the US are diagnosed with major depressive disorder, and most are treated with antidepressants such as Prozac, Zoloft or Paxil.

The brain chemical targeted by these drugs-called serotonin-is involved in a host of biologic functions, from mood, to attention, to appetite and general brain development.

While medicines that ratchet up serotonin levels help dampen depressive symptoms, it is not well understood how a human fetus reacts to such drugs, or how long their potential effects last. In the lab, for example, scientists have found that antidepressants given to a pregnant rat stifle the natural exploratory behavior of her offspring well into adulthood.

For their study, published in the journal Pediatrics, Pedersen and his colleagues tapped into a nation-wide Danish database of more than 100,000 pregnancies.

They identified some 400 women who took antidepressants during pregnancy as well as nearly 500 who were not on medication despite being depressed. Based on the women’s own reports, the researchers then compared how many children in each group hit developmental milestones such as sitting without support, looking after sounds and venting irritation.

At six months, the only differences between babies were seen in their gross movements. Among babies exposed to antidepressants in the second or third semester, 26 percent were able to sit on their own, compared to 30 percent of those not exposed.

The exposed toddlers took an average of 16 days longer to learn how to sit, after adjusting for maternal age, breast feeding and other factors. They also started walking about 29 days later.

At 19 months, the movement differences had vanished, although the exposed children were slightly worse at occupying themselves without calling out for attention.

Despite the concerns raised by these findings, which add to earlier reports of increased pain sensitivity and risk of heart problems in babies exposed to antidepressants in the womb, experts say that pregnant women with depression should not necessarily avoid antidepressants.

First, the new study could not say whether women who took medicine had been more depressed at first, even if they turned out to have fewer symptoms after treatment.

Second, untreated depression in itself has been tied to infant health problems such as irritability and lack of attentiveness. And third, the mother’s well being is at stake, too.

“This paper adds to a growing literature that prenatal antidepressant exposure is not risk free,” developmental pediatrician Dr. Tim Oberlander of the University of British Columbia told Reuters Health. He added that although there are many treatment options available, including psychotherapy, “there are going to be women who do need medication.”

“It’s really a question of balancing benefits and risks to the mother and child,” he said.

The latest guidelines from The American College of Obstetricians and Gynecologists and the American Psychiatric Association recommend that pregnant women consult an ob-gyn and a psychiatrist before deciding on treatment.

Pedersen said he backed those recommendations, but that he would like to see more large studies exploring how exposed babies fare later in life. His team plans to do one such study once the Danish kids, who are now 12 years and younger, reach their teens.

SOURCE: Pediatrics, February 22, 2010.