Antibiotics can double the risk of miscarriage if taken during early pregnancy, a new study suggests.
A major review involving more than 95,000 women has found five common classes of the drug were associated with an increased risk, while two others were shown to be safe.
Experts say the major risk highlighted by the study is for women prescribed antibiotics who do not yet know they are pregnant, as UK doctors are generally cautious in prescribing the medicines when they know conception has occurred.
Published in the Canadian Medical Association Journal, the investigation found a heightened chance of miscarriage was associated with commonly used antibiotics known as macrolides, quinolones, tetracyclines, sulfonamides and metronidazole.
But the scientists also found Erythromycin and nitrofurantoin, often used to treat urinary tract infections in pregnant women, were not associated with an increased risk.
Dr Anick Berard, from the Faculty of Pharmacy at the Universite de Montreal, said: “Infections are prevalent during pregnancy.
“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 60 per cent to two-fold increase.”
Roughly one in four pregnancies end in miscarriage meaning women prescribed some of the antibiotics flagged in the study could be raising their risk to a 50 per cent chance.
“Given that the baseline risk of spontaneous abortion can go as high as 30%, this is significant,” said Dr Berard.
“Nevertheless, the increased risk was not seen for all antibiotics, which is reassuring for users.”
Berard and her team looked at data from the Quebec Pregnancy Cohort between 1998 and 2009, comparing 8702 cases defined as clinically detected spontaneous abortions with 87 020 controls.
All the participants in the trial were between 15 and 45 years old.
The Miscarriage Association said the scale and quality of the study made it “a really important piece of research.”
Dr Nicola Davies, a GP and former trustee of the association, said: “The main risk this research throws up is for those women who don’t know they are pregnant.
“Most of these drugs are drugs you wouldn’t prescribe if you knew a woman was pregnant.”
Macrolides are commonly prescribed for respiratory infections, as well as some intestinal infections and skin infections, especially for those resistant to penicillin.
Meanwhile tetracyclines can be prescribed for people suffering from acne.
She said the data indicating that nitrofurantoin was safe was not associated with an increased risk of miscarriage was to be welcomed as until recently its safety had been in doubt.
Age is one of the most significant factors connected with the risk of miscarriage, with chances ranging from one in 10 for pregnant women under the age of 30 to 50 per cent in those above the age of 45.