Canadian researchers are raising the alarm about the use of antidepressants during pregnancy, unveiling research that finds the drugs are strongly linked to an increased risk of autism.

The new research found that women taking antidepressants during the second or third trimester of pregnancy had almost double the risk of having a child who would be diagnosed with autism by age 7.

The risk was particularly high when the mother took the most commonly prescribed antidepressants called selective serotonin reuptake inhibitors, or SSRIs. This class of drugs includes Paxil, Prozac, Zoloft, and others.

Prof. Anick Berard, of the University of Montreal and CHU Sainte-Justine children's hospital, specializes in the safety of medication in pregnancy and led the study.

Berard and her team looked at data from the Quebec Pregnancy Cohort, focusing on more than 145,000 children up to age 10. The databank recorded several categories of information, including the mother's age at birth, her history of depression and use of antidepressants, the child's family history of autism, the family's socio-economic status, and other factors.

The research team looked at which mothers had had one or more prescriptions for antidepressants filled during their second or third trimesters of pregnancy – a time that is a critical period for fetal brain development.

They then looked at which children had been diagnosed with autism, atypical autism, Asperger's syndrome, or a pervasive developmental disorder.

Among the 145,000 children, 1,054 children were diagnosed with autism by 4.5 years of age.

Mothers who had taken antidepressants during their pregnancies had an 87 per cent increased risk of having a child diagnosed with autism compared to those who didn't use the drugs, the researchers concluded.

The full results are published in JAMA Pediatrics.

Berard says the findings are important, since as many as six to 10 per cent of pregnant women are currently treated for depression with antidepressants.

As well, studies suggest that the prevalence of autism has increased in recent decades, from 4 in 10,000 children in 1966, to 100 in 10,000 today. That increase could be attributed to better detection and widening criteria for diagnosis, but the Montreal researchers believe that environmental factors are also a factor.

It's not clear why antidepressants might be linked to autism, but Berard says it's well-known that the drugs cross the placenta. She says the drugs block the absorption of serotonin, which is involved in numerous brain developmental processes, including cell division and differentiation, and by inhibiting serotonin absorption. She speculates that SSRIs and some other antidepressants could be hurting the ability of the brain to fully develop in the womb.

While there have been other studies linking antidepressants in pregnancy to autism risk, there have also been studies that uncovered no significant risk of the drugs. But Berard says this study used a very large sample size, focusing only on late pregnancy use of the drugs.

Because this was an observational study, Berard said her team could not determine whether antidepressants are causing autism, and she noted there is still a lot that experts don't know about the condition.

"The causes of autism are still not well understood," Berard told CTV News. "There's probably a strong genetic component… there is also probably a strong epigenetic effect, meaning an effect of genetics combined with the environment. Here, we've looked at one environmental exposure: the antidepressants."

Absolute risk still ‘small’: expert

Dr. Evdokia Anagnostou said that while the the Quebec study is well-designed, it should not cause panic among expecting mothers, as the absolute risk still remains small. Anagnostou is a Canada Research Chair in translational therapeutics in autism, and a senior clinician scientist at the autism research centre at Holland Bloorview Kids Rehabilitation Hospital.

Overall, she said, 1.2 per cent of the mothers in the study who used SSRIs had children who eventually developed autism, and this percentage falls within the established ranges found in the general population.

"The numbers that are being reported are not out of range," she told CTV News, noting that it's "not clear" that there is a causal relationship between antidepressants and autism.

She also said that there are risks in not treating depressed mothers, which may also lead to poor health outcomes for the baby.

"Depression during pregnancy is not a good thing," she said. "Typically, we don't want to leave moms who are depressed untreated, because there is also an association between depression and poor neuro-developmental outcomes."

Dr. Anagnostou said that pregnant mothers who are at risk of depression should talk with their doctors about the risks and benefits of taking antidepressants. However, she noted that it's important to consider the well-being of the mother, as well as the child.

"It's important to remember that moms count too," she said. "So the question becomes: 'Is it better to treat the depression or is it better to not treat the depression?'"

Berard also acknowledged that depression can be a serious condition that can lead to several other health problems. But she said, for those with milder forms of depression, it may be advisable to consider other treatment options during pregnancy.

"The majority of pregnant women who are depressed are mildly to moderately depressed and for sure, antidepressants are a treatment option but they are not the only one," she said.

There has been good evidence that psychotherapy and regular exercise and can also help.

But more importantly, she says women who are depressed need to plan their pregnancies and inform themselves of the possible risks of the medications they are taking.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip