Montreal—Which antibiotics have the best safety profile for treating infections during pregnancy? A new study in the Canadian Medical Association Journal offers some answers.

The Université de Montréal–led study cautions that many classes of common antibiotics, including macrolides, quinolones, tetracyclines, sulfonamides, and metronidazole, are linked to increased risks of miscarriage in early pregnancy.

On the other hand, neither erythromycin nor nitrofurantoin, often used to treat urinary tract infections in pregnant women, fall into that category, according to the report.

“Infections are prevalent during pregnancy,” explained lead researcher Anick Bérard, PhD, of the Faculty of Pharmacy. “Although antibiotic use to treat infections has been linked to a decreased risk of prematurity and low birth weight in other studies, our investigation shows that certain types of antibiotics are increasing the risk of spontaneous abortion, with a 60% to two-fold increased risk.”

To determine that, the study team reviewed data from the Quebec Pregnancy Cohort between 1998 and 2009, matching 8,702 clinically detected spontaneous abortions with 87,020 controls. The mean gestational age at the time of miscarriage was 14 weeks of pregnancy, and 16.4% of the spontaneous abortions were in women who had been exposed to antibiotics during early pregnancy compared to 12.6% of the controls.

The study controlled for issues raising the risk of miscarriage, including older age, living alone, and multiple health issues and infections.

Results indicate that use of azithromycin, adjusted odds ratio (AOR) 1.65; clarithromycin (AOR 2.35); metronidazole (AOR 1.70); sulfonamides (AOR 2.01); tetracyclines (AOR 2.59); and quinolones (AOR 2.72) were associated with an increased risk of spontaneous abortion. Study authors said similar results were found with penicillins or cephalosporins as the comparator group.

“After adjustment for potential confounders, use of macrolides (excluding erythromycin), quinolones, tetracyclines, sulfonamides and metronidazole during early pregnancy was associated with an increased risk of spontaneous abortion,” the researchers conclude. “Our findings may be of use to policy-makers to update guidelines for the treatment of infections during pregnancy.”

“Given that the baseline risk of spontaneous abortion can go as high as 30%, this is significant. Nevertheless, the increased risk was not seen for all antibiotics, which is reassuring for users, prescribers and policy-makers,” Bérard added.

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