Babies born to women prescribed trimethoprim-sulfamethoxazole in early pregnancy have a 35% higher risk of congenital abnormalities than those exposed to beta-lactams, a US study suggests.

Researchers have compared the risk of congenital malformations across more than 71,600 live births following first-trimester exposure to antibiotics.

Specifically, they compared malformation rates from pregnancies exposed to trimethoprim-sulfamethoxazole (TMP-SMX), nitrofurantoin and fluoroquinolones to those exposed to beta-lactams, known to be safe in pregnancy.

All the women (median age 30) included in the study were exposed to only one class of antibiotic, according to the authors from Washington University School of Medicine in St Louis, Missouri.