Women who smoke while pregnant contribute to the severity of asthma and poor lung function in their children, warns a study.

The findings published in the journal CHEST suggest that tobacco smoke exposure during pregnancy is more strongly associated with worse lung function than current, ongoing exposure in school-aged children with asthma. "This study implicates maternal smoking in pregnancy as the period of second-hand exposure that is more strongly associated with worse lung function in asthmatic children," said lead investigator Stacey-Ann Whittaker Brown from Icahn School of Medicine at Mount Sinai, New York.

"Maternal smoking in pregnancy may set children with asthma on a trajectory of poor lung function in later childhood, and other studies suggest this effect may be lifelong," Whittaker Brown said. Investigators analysed the relationship between lung function and the type of second-hand smoke exposure in a representative sample of school-aged children aged six to 11 years. The sample consisted of 2,070 children who participated in the 2007-2012 National Health and Nutrition Examination Survey (NHANES) in the US.

Detailed information about ongoing second-hand smoke exposure as well as parental self-reported exposure prior to birth was obtained. During the study period, lung function was measured using spirometry, and exposure to smoking was assessed through levels of cotinine in the blood, a marker of the extent of current second-hand smoke exposure. Thus, investigators were able to distinguish clearly between exposure in pregnancy and ongoing second-hand smoke exposure.

Nearly 10 per cent of both children with and without asthma in the sample had reduced lung function. Investigators found that current tobacco smoke exposure was independently associated with airflow obstruction in school-aged children, although the extent of the association was small. However, prenatal tobacco smoke exposure was associated with a 2.5 times increase in odds of having airflow obstruction in children with asthma, the study said.