The use of one class of antidepressants during pregnancy has been linked to a possible increase of the risk of autism in boys, a study suggests.

Researchers at the Johns Hopkins Bloomberg School of Public Health in Baltimore say their study found autistic boys were three times as likely to have had mothers who took antidepressants identified as SSRIs -- including Prozac, Celexa, Lexapro, Zoloft and Paxil -- during their pregnancy.

They are also more likely to display evidence of developmental delays, the researchers reported in the journal Pediatrics.

"We found prenatal SSRI exposure was almost three times as likely in boys with autism spectrum disorders relative to typical development, with the greatest risk when exposure is during the first trimester," says co-author of the study Li-Ching Lee.

While their study suggested a possible association between the prenatal use of antidepressants and the autism risk to boys, it was not proof of cause-and-effect, the researchers emphasized, adding there are known risks to both fetus and mother if depression remains untreated.

SSRIs taken during pregnancy penetrates the placenta and ups the fetus' level of serotonin, the researchers said. Serotonin has been known as the "happy hormone" that helps humans battle depression.

Roughly 33 percent of kids diagnosed with autism have abnormal levels of serotonin, which may be involved in the surfacing of certain autism symptoms, the researchers noted. 

Antidepressants are prescribed in about 4 percent of pregnancies, the researchers say.

"It's a complex decision whether to treat or not treat depression with medications during pregnancy," Lee says. "There are so many factors to consider. We didn't intend for our study to be used as a basis for clinical treatment decisions. Women should talk with their doctors about SSRI treatments."

Still, experts say, the overall risk of a child being born with autism remains at a very low level.

"If the risk of autism is around 1 percent now, and you raise it to 3 percent, that still means that 97 percent of the time, you won't have an autism spectrum disorder," says Dr. Eric Hollander, director of the autism and obsessive-compulsive disorder program at Montefiore Medical Center in New York City.  "The chances are still overwhelming that they won't have a child with an autism spectrum disorder."