A new study suggests that guidelines for antibiotic use during early pregnancy might need to be reviewed, after finding that some common classes of the drug could double the risk of miscarriage.

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Researchers have identified a link between antibiotic use in early pregnancy and greater risk of miscarriage.

Researchers from the University of Montreal in Canada have identified a link between the use of macrolides, quinolones, tetracyclines, and some other common antibiotics in early pregnancy and an increased risk of miscarriage.

The findings were recently reported in the Canadian Medical Association Journal.

Miscarriage is defined as the spontaneous loss of a fetus before 20 weeks of pregnancy.

According to the Eunice Kennedy Shriver National Institute of Child Health and Human Development, miscarriage occurs in around 15 to 20 percent of women who are aware of their pregnancy, and it is most common among women aged 35 or older.

Over half of all miscarriages are caused by abnormalities in the chromosomes of the fetus. Other factors that may raise the risk of miscarriage include diabetes, polycystic ovary syndrome, obesity, and infection.

Study co-author Dr. Anick Bérard, of the Faculty of Pharmacy at the University of Montreal, notes that antibiotics are widely used during pregnancy to help treat infection, but studies assessing their safety have produced conflicting results.

While previous studies have identified a link between antibiotic use in pregnancy and risk of miscarriage, the researchers note that these studies have a number of limitations, such as small samples and recall bias.

Dr. Bérard and colleagues sought to address such limitations with their new research. “We aimed to quantify the association between exposure to antibiotics during pregnancy and the risk of spontaneous abortion, taking into account methodologic limitations of previous studies,” they explain.

The researchers reviewed data from the 1998-2009 Quebec Pregnancy Cohort, which included 8,702 women aged between 15 and 45 years who had experienced a miscarriage at a mean gestational age of 14 weeks. These women were matched to 87,020 controls.

Antibiotic exposure in early pregnancy was identified among 1,428 (16.4 percent) women who had a miscarriage and 11,018 (12.6 percent) controls.

The researchers found that the use of certain antibiotics in early pregnancy increased the risk of miscarriage by as much as twofold.

The use of macrolides – excluding erythromycin – in early pregnancy was associated with increased miscarriage risk, as were quinolones, tetracyclines, sulfonamides, and metronidazole.

Nitrofurantoin – an antibiotic commonly used to prevent and treat urinary tract infections – was not linked to a greater miscarriage risk.

The team notes that the study sample size, valid data on filled prescriptions for antibiotics, and regularly collected information on miscarriage diagnoses are some of the key study strengths.

However, the researchers say that they are unable to rule out the fact that infection severity may have contributed to the miscarriages identified, which is a significant limitation.

Still, they believe that their findings question the safety of antibiotic use in early pregnancy. They conclude:

Our findings may be of use to policy-makers to update guidelines for the treatment of infections during pregnancy.”

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