Taking certain types of antibiotics during early pregnancy may increase the risk of miscarriage among pregnant women, according to a recent study published in the Canadian Medical Journal, CNN.com reports.

More specifically, researchers at the Université de Montréal in Canada suggested that several classes of bacteria-fighting drugs, including sulfonamides (for urinary tract infections, ear infections, bronchitis and eye infections), metronidazole (used to treat infections of the vagina, skin, stomach and respiratory tract) and macrolides (a common second-line antibiotic prescribed in place of penicillin), could increase the risk for pregnancy complications when taken in the first trimester.

For the study, scientists reviewed data from the medical records of thousands of women involved in the Quebec Pregnancy Cohort between 1998 and 2009. All patients ranged in age from 15 to 45 years old and were covered under the province’s health insurance plan. In the cohort, researchers noted 8,702 cases of diagnosed spontaneous abortions and compared these with 87,020 pregnancies in which a miscarriage did not occur.

Findings showed that, of the women who miscarried, 16.4 percent took an antibiotic during early pregnancy compared with 12.6 percent of women who didn’t take them. Scientists also noted that when a woman didn’t take antibiotics during pregnancy, her risk of a miscarriage was about 6 to 7 percent. That risk increased to 10 percent, however, if she used certain antibiotics. 

But researchers also found that antibiotics such as penicillin, nitrofurantoin (another drug that treats urinary tract infections) and erythromycin (prescribed for respiratory and skin infections as well as several sexually transmitted infections) were considered safe for pregnant women.

“It is reassuring to see that first-line treatments and antibiotics that are the most used in the population were not associated with an increased risk of miscarriage,” said Anick Bérard, PhD, a member of the Faculty of Pharmacy at the Université de Montréal, and the lead author of the study. 

In addition, scientists noted that women who miscarried were more likely to be older, live alone or suffer from multiple health issues and infections — factors accounted for when researchers calculated the increased risk for moms-to-be taking antibiotics.

Overall, the benefits of antibiotic treatment largely outweighed the risks, scientists said, especially since infections may also contribute to miscarriage during pregnancy. But the big takeaway here is that these findings may change the way doctors prescribe the germ-fighting agents to expecting mothers. 

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