For the new study, Berard and colleagues tapped into data collected on all pregnancies in Quebec between 1998 and 2009. Among more than 145,000 full-term, singleton children, they could then compare how outcomes varied depending on the kinds of drugs prescribed to pregnant women and their stages of pregnancy at the time.
Antidepressant use during the first trimester made no difference in autism diagnoses down the line, the researchers report today in the journal JAMA Pediatrics. But when women took antidepressants during their second or third trimesters, their children were 87 percent more likely to be diagnosed with autism by age 7.
Risks doubled among children of women who took SSRI's, such as Paxil, Prozac and Zoloft. When women took more than one class of drugs, risks were four times as high.
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Although this study looked only at correlations, previous research has linked autism with abnormalities in the brain's ability to make serotonin, a neurotransmitter that affects mood, behavior and more. SSRIs (which block the transport of serotonin and increase levels of it in the brain) cross the placenta and could alter fetal brain development, particularly during the second and third trimesters.
But there are plenty of caveats against over-interpreting the new results. For one thing, total numbers were small. With antidepressant use during the later stages of pregnancy, the risk of autism rose from 0.7 percent of births to 1.2 percent. That adds up to 12 extra children out of more than 2,500 who might have been able to avoid the diagnosis if their mothers hadn't taken medication while pregnant.
But still, scientists can't say for sure if it was the drugs that made the difference, says Bryan King, director of the Seattle Children's Autism Center at the University of Washington and Seattle Children's Hospital.
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Perhaps the women who were able to successfully stop taking antidepressants during pregnancy had a different kind of depression, and maybe something about their disease independently influenced autism risk. A family history of depression is also known to increase risk for autism.
And still, most women who took the drugs had children without autism, suggesting that antidepressants are only a small part of a much bigger, more complicated and multifaceted story.
The new study "continues to add to a body of work that suggests that as we're looking for potential environmental contributions to autism, that the prenatal time period is an important place to look," King says. "Whether the findings should actually change the way we behave, I'm not sure they should."
In his view, the small number of children affected should actually be a reassuring sign.
"You could look at this and take comfort in that the risk is not actually huge," he adds. "The last thing I would want is for people to feel compelled to stop taking an effective antidepressant because someone is suggesting antidepressants cause autism."