Background: Prenatal smoke exposure is associated with airway inflammation and asthma in children. It also increases the risk of low birth weight (LBW). LBW is associated with decreased lung function independently of smoking.
Objective: To study the independent and joint effects of prenatal smoking and LBW on childhood asthma.
Methods: In 1996, all children aged 7 to 8 years in 3 cities in northern Sweden were invited to an International Study of Asthma and Allergy in Childhood questionnaire survey. This study focused on the follow-up of children aged 11 to 12 years, in which 3389 children (96%) participated. A subset of 2121 children underwent skin-prick testing. Self-reported physician-diagnosed asthma has been clinically validated.
Results: Mean birth weight was 3360 g in children exposed to prenatal smoking and 3571 g in nonexposed children (P < .001). The association of prenatal smoking with physician-diagnosed asthma was stronger in LBW children (risk ratio: 8.8 [95% confidence interval: 2.1-38]) than in normal birth weight children (risk ratio: 1.3 [95% confidence interval: 1.0-1.8]). LBW alone was not an independent predictor of asthma. These associations were similar in multivariate analysis, and the interaction term LBW × smoking was highly statistically significant.
Conclusions: There was a strong interaction of LBW and prenatal-smoking on the risk of physician-diagnosed asthma, which has not been demonstrated previously. This was consistently seen with adjustment for known risk factors, including allergic sensitization. Plausibly, airway inflammation from prenatal smoke exposure induces obstructive symptoms more easily in the underdeveloped airways of LBW children.