London – Common antibiotics taken in pregnancy raise the risk of birth defects in babies, a study suggests.
It found macrolides – a group of antibiotics often used to treat chest and urinary tract infections – significantly increase the chances of disorders of the heart, genitals and brain.
While most antibiotics, such as penicillin, are safe during pregnancy, the study raises new concerns about the macrolides group, which includes common antibiotics such as erythromycin, clarithromycin and azithromycin.
These treatments have been given to about three percent of pregnant women in Britain over the past 26 years, according to the study by researchers at University College London.
A total of 31 percent of mothers take antibiotics in pregnancy, with 21 percent using penicillin. But macrolides are among the most frequently prescribed for the many who are allergic to penicillin.
The research team, whose work is published in the British Medical Journal, found babies whose mothers took macrolides in the first trimester of pregnancy were about 55 percent more likely to be born with a defect than those whose mothers took penicillin.
Some macrolides come with warnings that they should not be taken in pregnancy. But others – including erythromycin, the most commonly used macrolide – are listed as "not known to be harmful" in pregnancy.
Researchers used a database of just over one million children born in the UK from 1990 to 2016, tracked until the age of six. Of those, 105 000 had mothers who had taken macrolides or penicillin during pregnancy.
Major malformations were present in 186 of the 8,632 children whose mothers were prescribed macrolides at any point during pregnancy and 1,666 of the 95,973 whose mothers were prescribed penicillin during pregnancy.
After adjusting the figures for other factors that could influence the findings, the researchers calculated that 28 in every 1 000 babies whose mothers took macrolides during the first three months of pregnancy developed birth defects.
Macrolides led to a risk 55 percent greater than for penicillin, which resulted in 18 defects per 1 000 births, roughly the same general risk that any baby will be born with a health defect.
While the greatest risk was seen in the first trimester, taking macrolides at any point in pregnancy pushed up the risk of birth defects by 23 percent when compared with penicillin.
Professor Ruth Gilbert, a researcher at the UCL Great Ormond Street Institute of Child Health, said: "Our findings suggest it would be better to avoid macrolides during pregnancy if alternative antibiotics can be used."
But she added: "Women should not stop taking antibiotics when needed, as untreated infections are a greater risk to the unborn baby."
The team stressed that the size of the risk remains very low.