The association was weak, but juries won't know that, because Dr. Anick Bérard, Faculty of Pharmacy, Université de Montréal, declared, "our investigation shows that certain types of antibiotics are increasing the risk of spontaneous abortion, with a 60% to two-fold increased risk." That's important, since baseline risk of spontaneous abortion is 30 percent, but the women who miscarried in this study were more likely to be older, living alone and to have multiple health issues and infections.
They say they tried to account for that, and drew their conclusion by looking at data from the Quebec Pregnancy Cohort between 1998 and 2009. For this study, 8702 cases, defined as clinically detected spontaneous abortions, were matched with 87,020 controls; mean gestational age at the time of miscarriage was 14 weeks of pregnancy. A total of 1,428 (16.4%) cases were exposed to antibiotics during early pregnancy compared to 11,018 (12.6%) in controls. Participants were between the ages of 15 and 45 years and covered under Quebec's drug insurance plan.
A large sample, valid information on filled prescriptions and routinely collected information on diagnosis of spontaneous abortion or related procedures are all good aspects. But infection severity is a huge confounder that could explain the increased risk. The authors say they tried to adjust for that in the analysis but 10 different statistics experts would have 10 different conclusions on how well they did.
Erythromycin was not associated with increased risk nor was nitrofurantoin, often used to treat urinary tract infections in pregnant women.