Canadian researchers are raising new worries about the use of antidepressants in pregnancy, after finding that the medications appear to hike the risk of miscarriage by 68 per cent.

The study could leave those treating pregnant women with depression with some tough decisions to make about treatment, as they weigh the risks of the medications against the risks of not treating the depression.

It's estimated that up to 3.7 per cent of pregnant women will use antidepressants at some point during their first trimester. Previous studies on the risks of the drugs during pregnancy have led to contradictory findings.

Some studies have shown that the medications do not increase the risk of birth defects. But how the medications might affect miscarriage rates specifically has not been well studied, the authors note.

So for this study, published in the Canadian Medical Association Journal, researchers from the University of Montreal and CHU-Sainte-Justine hospital decided to find out how different kinds of antidepressants might affect miscarriage rates.

They reviewed the cases of more than 5,000 pregnant women from Quebec who lost their unborn babies before the 20th week of pregnancy. They compared those women with pregnant women from the same registry who didn't have miscarriages.

Of those who miscarried, 5.5 per cent had filled at least one prescription for antidepressants during their pregnancy. That compared with 2.7 per cent of the control group who didn't have miscarriages.

The researchers found that one class of antidepressants in particular -- selective serotonin reuptake inhibitors, or SSRIs – was associated with the strongest link to miscarriage. Among that class, paroxetine (often sold as Paxil) and the SNRI venlafaxine (sometimes sold as Effexor) appeared to be most strongly linked to an increased risk of miscarriage.

As well, they found that a combination of different antidepressants doubled the risk of miscarriages.

The researchers say their findings are significant, especially considering how large the study was.

The problem now is what it means for patients and their doctors.

While the findings are worrisome, there are also worries that come with asking pregnant patients to stop taking antidepressants. Discontinuing treatment can lead to a relapse in depression, which can put mother and baby at risk, the study authors note. And there is some evidence that depression itself if harmful to the baby.

"In light of our results, physicians who have patients of childbearing age taking antidepressants or have pregnant patients who require antidepressant therapy early in pregnancy may wish to discuss the risks and benefits with them," the authors conclude.