Lactating mothers who use e-cigarettes or nicotine replacement therapies may be putting their breastfed babies at risk for skull defects, a new study in animals suggests.

Cigarette smoking has already been linked to increased risk for these abnormalities in previous research. This study tested the effects of nicotine alone on head and face development. Researchers added nicotine to the drinking water of adult female mice that were nursing litters of newborn pups. The nicotine exposure was the equivalent of about one-half to a full pack of cigarettes per day.