Women who take nonsteroidal anti-inflammatory drugs, or NSAIDs, during the first stage of their pregnancy may run a 2.4 increased risk of miscarriage, according to a new study that examined medical records of women who filled at least one prescription for such pills as ibuprofen, naproxen and Celebrex. Over-the-counter NSAIDs were not included.

The study, which was published in the Canadian Medical Association Journal, compared records of 4,705 women who suffered a miscarriage during the first 20 weeks of pregnancy with records of 47,050 women who became pregnant and ultimately delivered a child. All of the women in the study were between the ages of 15 to 45 when they became pregnant (read the abstract here).

Among women who suffered miscarriages, 352 had taken an NSAID, compared with 1,213 of the women who did not experience pregnancy loss. The study was not a randomized, controlled trial, but the researchers believes the results are due to the pills and cautioned that “gestational exposure to any type or dosage of non-aspirin NSAIDs may increase risk of spontaneous abortion.”

“We went through a lot of pain and misery to make sure what we’re seeing is due to the drug and not a consequence of symptoms [of an underlying illness] or risk attributed to rheumatoid arthritis, for example,” Anick Berard, the lead author and director of the Research Unit on Medication and Pregnancy at St. Justine Hospital in Montreal, tells HealthDay.

To calibrate their findings and the risk associated with NSAID use, the researchers say they accounted for other factors that might increase the odds of miscarriage, including diabetes, high blood pressure, heart disease, asthma, lupus, rheumatoid arthritis, depression and anxiety.

As to specific drugs, different risks were found. The highest risk was associated with diclofenac, which tripled the risk. Naproxen yielded a 2.64-fold risk and ibuprofen carried an increased risk of 2.2 fold, as did Celebrex, which is sold by Pfizer. The findings suggest a class effect, according to the researchers. The mechanism is unclear, but they speculate NSAIDs may interfere with levels of hormones called prostaglandins, which are involved in inducing labor, HealthDay writes.

“We cannot say for 100 percent sure that this is a true drug effect,” Berard tells 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.”