Common anti-inflammatory drugs like ibuprofen and aspirin may increase a woman's risk of miscarriage, researchers found in a new study published Tuesday in the Canadian Medical Association Journal.

The findings concerned non-steroidal anti-inflammatory drugs, or NSAIDs, the most common of which are ibuprofen (Advil or Motrin), aspirin and naproxen. They do not apply to acetaminophen, or Tylenol, a non-NSAID painkiller that is safe for pregnant women.

We're seeing a 2.4-fold increased risk in spontaneous abortion following early use of NSAIDs during pregnancy, Anick Berard, an epidemiology professor at the University of Montreal and the senior author of the study, told HealthDay.

The risk was strongest for women who took NSAIDs during the first 20 weeks of pregnancy. However, the drugs are known to be dangerous in the third trimester as well, for a different reason: they can cause fetal circulation problems.

I would strongly suggest that women take no non-steroidal anti-inflammatory drugs during the first trimester, Berard told MSNBC. If a woman is taking an NSAID for a chronic condition, she really has to talk to her health care provider to see if it's feasible to stop at least during the first trimester.

The findings may not be conclusive because the study was not randomized or controlled, leaving the possibility that the increased risk of miscarriage among women who took NSAIDs could be the result of an unrelated factor that the women had in common. However, the researchers said they had accounted for a number of such factors -- diabetes, hypertension, heart disease, asthma, lupus, rheumatoid arthritis, depression and anxiety -- while calculating their results. 

The study looked about 5,000 women who had miscarried and 50,000 who had not miscarried. They found that 352 of the 5,000 women who miscarried (7 percent) had taken NSAIDs during the first 20 weeks of pregnancy, while just 1,213 of the 50,000 women who did not miscarry (2.4 percent) had taken the drugs. Among the women who took NSAIDs, about 35 percent miscarried -- much higher than the 15 percent rate of miscarriage in the general population.

Taking the potential confounding variables into account, the researchers calculated that women who had filled an NSAID prescription just before becoming pregnant or anytime during the first 20 weeks of pregnancy had a 2.43 times greater chance of miscarrying. Berard said she thought the results would be similar for women who used over-the-counter NSAIDs, which come in lower doses.

She speculated that NSAIDs might interfere with levels of prostaglandins, which help induce labor.

We cannot say for 100 percent sure that this is a true drug effect, Berard told MSNBC. But we're one step closer to proving causality when there is repetition of the finding, and there is at least one other study looking at this specifically that found an increased risk.