Women taking the anti-diabetes medication Ozempic might have an extra reason to pat their tummies. Not only does the injectable drug lead to weight loss, a finding that has sparked frenzied off-label demand worldwide, but in recent months it has also been linked to a surprise baby boom.

Now scientists are trying to unpick the mechanism behind the “Ozempic babies” phenomenon, which has been reported by users of similar medications, too. This matters: the drugs were never tested for use by women who were pregnant or trying to conceive, and animal studies on Ozempic’s key ingredient, semaglutide, suggest an association with birth defects.

Today, the number of people taking weight-loss medications makes this an urgent health issue. While they are prescribed for diabetes, obesity and weight-related health complications, they also appear to improve heart and kidney health. That has led to calls for more widespread prescribing, for drugmakers to slash prices and for insurers to pick up the tab. But it feels presumptuous to regard the medications as indispensable when their long-term impact, such as on the health of babies born to users, remains unknown.