Lactating mothers who use e-cigarettes or nicotine replacement therapies may be putting their breastfed babies at risk for skull defects, according to a new animal study published in the journal FASEB.

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. Scientists found in 15-day-old pups that the skull joints across the top of their heads were narrowed, putting them on a path to fuse earlier than normal. Because mouse pups at this age don’t drink water, breast milk was the only possible source of their nicotine exposure.

In human babies, this skull abnormality not only changes the shape of the head but can require neurosurgery to make room for the brain to grow, according to the researchers.

The study builds upon previous work by the Ohio State University researchers that showed in mice that nicotine exposure during pregnancy altered offspring’s craniofacial growth and development.

“We knew based on previous data in pregnancy that we’d see some changes, but we were a bit taken aback to find there were discernible differences when the nicotine exposure was occurring only during lactation,” said James Cray, associate professor of anatomy in Ohio State’s College of Medicine and senior author of the study.

“Our data suggest that nicotine alone can alter development of the head and face. That means mothers who vape are likely exposing their unborn children or infants to an amount of nicotine and its metabolites that can disturb growth in the same way cigarettes can.”

When the pups were 15 days old, which roughly equates to age 1-2 years in humans, the scientists used micro-CT scanning to measure their heads. They found abnormalities in development of the pups’ coronal sutures, joints that span the top of the head from ear to ear.