Smoking during pregnancy has been frowned upon for many years, but the exact mechanism linking smoking to higher risk for attention-deficit/hyperactivity disorder (ADHD) in children born to mothers who smoked during their pregnancies has remained elusive. A report published on Monday in the journal Pediatrics suggests that nicotine exposure or related factors may be to blame.

The authors noted that risk for ADHD was elevated for both children of mothers who smoked and for children of mothers who did not smoke but who used nicotine replacement therapy, including nicotine patches. They stress, however, that their findings do not prove that ADHD is caused by nicotine.

Researchers at Aarhus University and the University of Copenhagen, Denmark, in collaboration with a colleague at the University of California in Los Angeles, studied 84,803 singletons (persons born with no siblings also born in the same birth) participating in the Danish National Birth Cohort. Records revealed whether and how much mothers smoked or used nicotine replacement during the pregnancies. Children with ADHD were identified from three Danish medical registries and the seven-year Cohort follow-up.

The researchers found that both maternal and paternal smoking during pregnancy led to higher risk of ADHD; the association was stronger for maternal smoking.  Children who were born to smoking mothers and non-smoking fathers had higher ADHD risk than children with non-smoking mothers and smoking fathers. Also, ADHD risk was higher in children of mothers who did not smoke but who used nicotine patches or other replacement during pregnancy, suggesting that nicotine specifically may increase risk of ADHD.

Smoking during pregnancy is known to cause low birth weight. The researchers found that women who had stopped smoking but used nicotine replacement gave birth to infants of higher, more healthy weights than did mothers who smoked during their pregnancies.

“The best advice will at this point probably be to try to stop smoking without use of nicotine replacement and preferably before getting pregnant,” said Carsten Obel, senior author of the report. “If this is not possible nicotine replacement is, based on the birth weight results, preferable in comparison with continuing to smoke.”