The use of certain kinds of common antibiotics early in a pregnancy appears to be linked to an increased risk of miscarriage, new Canadian research has confirmed.

The study, which appears in the Canadian Medical Association Journal, finds that antibiotics known as quinolones, tetracyclines, sulfonamides, macrolides are linked to miscarriage in early pregnancy.

But while these antibiotics may be unsafe during pregnancy, many others are considered safe for use in pregnancy under currently established Canadian guidelines.

These include penicillin, erythromycin, nitrofurantoin, and cephalosporin. None of these have been linked to birth defects, and in fact have been shown to reduce the risk of premature delivery, as well as low birth weight.

Antibiotic use is common in pregnancy. The good news is that the medications considered safest are also the ones most commonly prescribed in Canada for pregnant women to treat such things as urinary tract infections.

"So this is reassuring," lead researcher, Dr. Anick Bérard, from the Faculty of Pharmacy at l'Université de Montréal, told CTV News.

But the study confirms that certain classes of antibiotics should be avoided in pregnancy.

Her team looked at data from the Quebec Pregnancy Cohort between 1998 and 2009 but focused only on clinically confirmed pregnancies, meaning those that were far enough along to have been confirmed by a doctor. The mean gestational age at the time of miscarriage was 14 weeks.

They looked at 8,702 miscarriages and then matched them with 87,020 other pregnancies and found that the risk of miscarriage rose from approximately six per cent for those women not taking antibiotics in their pregnancies to nine to 12 per cent, depending on which antibiotic they took.

Overall, there were 1,428 miscarriages among women who took antibiotics during early pregnancy (16.4 per cent), compared to 12.6 per cent among 11, 018 controls who didn't take the antibiotics.

Berard says the results are in line with previous similar studies that have found miscarriage risks from certain antibiotics. But she notes her study used a larger sample size, focusing on a larger group of women.

"So really, our study is providing additional data on antibiotic use in pregnancy with regards to miscarriage," she said.

The research should not make pregnant women fearful to take antibiotics they are carefully prescribed by their doctors, said Dr. Deborah Money, a professor of obstetrics and gynaecology at the University of British Columbia.

"My biggest worry would be that women would not take needed therapy for serious infections and therefore put their health and their baby's health at risk," said Money, who is also an infectious diseases expert with the Society of Obstetrics and Gynecology of Canada.

"It's really important for all of us to remember that the safest thing for a developing fetus is a healthy mom, and if a mother has a serious or critical infection, that that is not a safe environment for that fetus, and we must be treating women properly for any serious illness they may have in pregnancy."

Many classes of common antibiotics, such as macrolides, quinolones, tetracyclines, sulfonamides and metronidazole, were associated with an increased risk of miscarriage.

They include these drugs:

  • Azithromycin
  • Clarithromycin
  • Ciprofloxacin
  • Spiramycin
  • Telithromycin
  • Norfloxacin
  • Levofloxacin
  • Moxifloxacin
  • Ofloxacin
  • Doxycycline
  • Minocycline
  • Tetracycline
  • Demethylchlortetracycline
  • Clindamycin
  • Metronidazole

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip