Children who are exposed to tobacco smoke from their fathers while they’re in the womb may be more likely to develop asthma by age 6, according to a study of chemical changes to DNA.

While prenatal smoke exposure has long been linked to an increased risk of childhood asthma, the current study offers fresh evidence that it’s not just a pregnant mother’s smoking that can cause harm.

Researchers followed 756 babies for six years. Almost one in four were exposed to tobacco by fathers who smoked while the child was developing in the womb; only three mothers smoked.

Overall, 31% of kids with fathers who smoked during pregnancy developed asthma by age 6, compared with 23% of kids without fathers who smoked, the study found.