A large study that tracked nearly 150,000 pregnancies in Canada over more than a decade has found that taking antidepressants during the second or third trimester could increase the risk of the child developing autism spectrum disorder ( ASD) by 87 percent.

And while that sure sounds scary, it's important to remember that this figure is relative to the risk that already exists for autism even before medications are involved, and there are multiple studies that have turned up no such link. "It kind of leaves you more confused," psychiatrist Alan Brown from Columbia University, who was not involved in the study, told NPR. "Mothers shouldn't get super worried about it."

For the study, researchers from the University of Montreal accessed information from the Québec Pregnancy/Children Cohort, which includes data on 145,456 pregnancies resulting in children born in Québec between 1 January 1998 and 31 December 2009. 

Three percent of these children were exposed to antidepressants during pregnancy - 89 percent during the first trimester, and 54 percent in the second and third (which covers months four to nine). Overall, 1,054 of the 145,456 children (0.7 percent) were subsequently diagnosed with ASD, the majority of them boys.

The highest risk group included the 2,532 babies whose mothers took antidepressants during their second and/or third trimester - 1.2 percent were diagnosed with autism.

After taking into account factors such as the socio-economic status of the family, family history of autism, and the age of the mother and her history of depression, the researchers found that the use of antidepressants known as selective serotonin re-uptake inhibitors (SSRIs), which includes Prozac, Zoloft and Celexa, was the strongest link in these higher risk children.

"Our study has established that taking antidepressants during the second or third trimester of pregnancy almost doubles the risk that the child will be diagnosed with autism by age seven, especially if the mother takes selective serotonin re-uptake inhibitors," lead researcher Anick Bérard told Steve Connor at The Independent.

"It is biologically plausible that anti-depressants are causing autism if used at the time of brain development in the womb, as serotonin is involved in numerous pre- and postnatal developmental processes, including cell division, the migration of neurons, cell differentiation and synaptogenesis - the creation of links between brain cells," she added.

The study has been published in the journal JAMA Pediatrics, but it's not without its critics. Of particular concern is that headline-ready "87 percent" figure. On its own, it sounds alarming, but when you consider that the risk of a child developing autism is already 1 percent, that makes the risk associated with SSRI's in this study 1.87 percent - small enough that it could have been an error in analysis. 

Then there are the previous studies that found no such link. As Jon Hamilton reports for NPR, a study earlier this year of several thousand children in the US found no link between antidepressant exposure and heightened autism risk, and a 2013 survey of nearly 670,000 children in Denmark came to the same conclusion. 

On the other hand, a 2013 study of more than 4,400 children in Sweden did find a link between the two, and back in 2011, a small study of 300 Californian children suggested that antidepressants could potentially increase the risk of autism "modestly". 

To say the jury's still out on this one is an understatement, and it has to do with the fact that figuring out the actual risks for autism is extremely difficult. In this scenario, the increased risk in children whose mothers have diagnosed depression could have nothing to do with the drugs themselves, but with the array of underlying factors that led to them developing depression in the first place.

While Bérard's study tried to take the mothers' depression into account, it did not factor in the severity of the depression, or the SSRI dosage. If your depression is severe, you're more likely to keep taking antidepressants into the second and third trimester, so it could be the severity of the depression that led to a higher risk of autism in this group rather than the drugs. Having said that, no study has convincingly linked depression to ASD.

NPR also points out that Bérard seems to have a bit of an agenda when it comes to antidepressants, reporting that she's done previous research linking their use to birth defects, and has consulted to plaintiffs who intend to sue the pharmaceutical companies that make them.

A larger follow-up study is now being conducted, the results of which will be available in the next few years. But until then, researchers have urged mothers not to panic, as not taking antidepressants during pregnancy could harm the baby in other, more substantiated, ways. 

"Depression is a serious disease," psychiatrist Bryan King from the autism centre at Seattle Children's Hospital, who wrote an editorial accompanying the study, told Sarah Zhang at Wired. "Untreated depression is associated with lack of nutrition or lack of sleep or fatigue and stress, which we know can cause problems in terms of foetal development."

"This study looked only at one developmental outcome and there is no control group that would enable us to capture all of the potential harm that might have been prevented with the choice to treat depression," he told Reuters. "It is important that women have full discussions with their health care providers about the complex interplay of risks and benefits associated with depression treatment."