Macrolide antibiotics taken during pregnancy may increase the risk of birth defects, researchers have warned.

A new study found that macrolides prescribed during the first trimester may increase the risk of birth defects, particularly cardiovascular malformations, and that drug safety labels should report this information.

The research, published in the BMJ, looked at data for almost 105,000 women who were prescribed either macrolide antibiotics or penicillin during their pregnancy, between 1990 and 2016.

Women who were prescribed a macrolide during the first trimester of pregnancy were 62% more likely than those prescribed penicillin to have a baby with a cardiovascular malformation.

A cardiovascular malformation occurred in almost 11 per 1,000 live births in the macrolide group, compared to almost five per 1,000 in those who were prescribed penicillin.

The researchers estimated that if the associations were causal, an additional four children per 1,000 who were exposed to macrolides instead of penicillin in the first trimester would be born with a cardiovascular malformation.

Macrolide prescribing in any trimester was also associated with a 58% increased risk of genital abnormality compared to penicillin.

The researchers did not find any significant risk of birth defects associated with macrolide prescribing specifically in the second or third trimester.

They said in the paper: ‘The increased risks of major malformations found in this study provide evidence that macrolide prescribing during pregnancy warrants caution.

‘Drug safety leaflets should report that there is uncertainty about the safety of macrolides, including erythromycin, and recommend use of alternative antibiotics when feasible until further research is available.’