A Canadian study of 8,700 women who had miscarriages has found a link between taking several classes of common antibiotics in early pregnancy and a 60% to 200% increased risk of miscarriage.

The antibiotics that were associated with the boost risk of miscarriage included macrolides (apart from erythromycin), quinolones, tetracyclines, sulfonamides and metronidazole.

The research, published in the Canadian Medical Association Journal, finds an association but the exact mechanism of how certain antibiotics could be increasing risk of miscarriage is not known. Only women who had miscarriages in early pregnancy were studied in this research.

However, the researchers urge caution in interpreting the results. It is possible that part of the increased risk is due to the infection for which the antibiotics are prescribed, rather than the antibiotics themselves, study author Anick Bérard of the University of Montreal told IBTimes UK.

"A medication is never prescribed in isolation – it is prescribed to treat a condition," Bérard said.

Without a clinical trial where some women are given antibiotics to treat an infection and others are not given antibiotics to treat that infection, the exact causal relationship cannot be definitely known. Such a study would not be ethical, as they would put both the expectant mother and foetus at risk.

"We still cannot completely rule out the fact that infection could explain part of our findings. It is reassuring that our findings [agree] with other studies and with the current guidelines, which state that quinolone and tetracyclin should not be used during pregnancy," Bérard said.

Not all antibiotics were associated with a higher risk of miscarriage. These could be working via a different mechanism of action to those that did carry the higher risk, the researchers say.

"We found that Penicillin, Cephalosporin, erythromycin – the most frequently used antibiotics – are not associated with an increased risk of miscarriage," Bérard said. "In addition, we found that quinolone and tetracycline are associated with an increased risk of miscarriage, which [agrees] with guidelines that say that they should not be used during pregnancy."

Sohinee Bhattacharya of the University of Aberdeen, a medical researcher who studies miscarriage and who was not involved in the research, warns that women who have an infection during pregnancy should still consult their doctor about whether antibiotics are appropriate.

"People shouldn't go away with the message that antibiotics are bad for all pregnant women. That would be completely the wrong. But having said that, I think we need further studies looking at antibiotic usage and the risk of miscarriage," said Bhattacharya.

The study authors agreed that women should not avoid antibiotics during pregnancy if their doctor advises them that they are necessary.

"The take home message is that infections need to be treated during pregnancy," said Bérard. "Women should seek medical counsel to identify the best treatment option.

"It is reassuring to see that first-line treatments and antibiotics that are the most used in the population – penicillin, cephalosporin – were not associated with an increased risk of miscarriage."