Taking antidepressants during pregnancy may slightly increase the risk of having a child with autism, but the chances of having a baby without the disorder are still about 98.8 per cent, new Canadian research suggests.

Children with autism spectrum disorder (ASD) may have difficulties with social communicating and show repetitive behaviour. It occurs in about one per cent of the general population worldwide.

The causes of autism spectrum disorder are mysterious. Some research points to genetic vulnerability, but genes alone don't offer a full explanation.

That's why investigators at the University of Montreal consider it a public health priority to try to understand the long-term effects of taking antidepressants in pregnancy on the neurodevelopment of children, given that 4.5 per cent of pregnant women in Canada have reported using antidepressants.

Anick Bérard, a pharmacy professor at the University of Montreal, is the latest to analyze data on births in Quebec between 1998 and 2009, antidepressant prescriptions from the province's public prescription drug insurance plan and medical claims for autism spectrum disorder.

"Use of antidepressants, specifically selective serotonin reuptake inhibitors, during the second and/or third trimester increases the risk of autism spectrum disorder in children, even after considering maternal depression," Bérard and her team said in Monday's issue of the journal JAMA Pediatrics.

Among mothers who filled at least one prescription for antidepressants during their second and/or third trimester, the risk of having an autistic child was 1.87 times higher.

Among those who tooks SSRIs such as fluoxetine (Prozac) or paroxetine (Paxil), the risk was 2.17 times higher.

A journal editorial by Dr. Bryan King of the department of psychiatry and behavioural sciences at the University of Washington provides the real numbers broken down by trimester.

Of the 2, 532 children exposed to the drugs during the second or third trimester, 31 had autism spectrum disorder — an additional 12 children affected with ASD than would otherwise be expected.

"Of course, even one potentially preventable case is noteworthy," King said. "But this finding must also be viewed in the context of other risks."

The findings are still within the range of autism prevalence reported across Canada and the U.S., said Dr. Evdokia Anagnostou, a senior clinician scientist at Holland Bloorview Kids Rehabilitation Hospital in Toronto.

Moms' wellness

"Moms need to understand that it is still a rare event, that we have more than a 98 per cent chance of having a typically developing child that doesn't have ASD," said Anagnostou, Canada research chair translational therapeutics in autism and an associate professor at the University of Toronto.

"When they speak to their physician, the discussion should be about the wellness of themselves, their own mental health and their ability to raise the child that's come into this world. Plus they should also know that untreated depression is also associated with somewhat higher number of poor neurodevelopmental outcomes."

While such studies measure a small association between taking antidepressants and risk of autism spectrum disorder, no cause-and-effect relationships can be drawn without doing a randomized trial, which is unethical in pregnancy.

Previous research has pointed to potential physical side-effects of taking some antidepressants during pregnancy. For example, Health Canada publicized GlaxoSmithKline's warning of possible heart defects in newborns associated with Paxil.

Bérard suggests all women with a chronic disease talk to their doctor before pregnancy, get informed about the risks and benefits of medication, and consider alternatives such as psychotherapy and exercise.

Anagnostou said the new findings are important for people who study biology, because they may encourage them to take another look at how changing serotonin during pregnancy may influence how the brain develops.

The question of whether it's the depression causing the problem or the SSRIs themselves remains unanswered.

What's known is that if a family has one baby on the spectrum, there's a 10 to 20 per cent chance another child will be affected because of genetic predisposition, Anagnostou said.

In the study, other classes of antidepressants were not associated with any statistically significant increased risk of autism spectrum disorder.

Too soon to change health policy

Dr. Peter Szatmari, chief of the Child and Youth Mental Health Collaborative between the Centre for Addiction and Mental Health, SickKids and the University of Toronto, said the Quebec study was based on a large sample size and accurate data on prescriptions.

However, some cases of autism spectrum disorder may have been missed, Szatmari said. The diagnoses weren't validated, and the researchers couldn't determine if the women in fact took the antidepressants prescribed.

He also questioned the biological basis of finding an association for the second and third trimester and not the first trimester.

In general, it's too early based on this one study to influence practice or policy, Szatmari said in an email provided to CBC News.

The study was funded by the Canadian Institutes of Health Reseach (CIHR) and the Québec Training Network in Perinatal Research.