Maternal Smoking in Pregnancy and Externalizing Behavior in 18-Month-Old Children: Results From a Population-Based Prospective Study

Objective: The relation between prenatal smoking and child behavioral problems has been investigated in children of school age and older, but prospective studies in younger children are lacking. Using the population-based prospective Norwegian Mother and Child Cohort Study, we examined the risk for externalizing behaviors among 18-month-old children after exposure to maternal smoking during pregnancy. Method: Participants were 22,545 mothers and their 18-month-old children. Mothers reported their smoking habits at the 17th week of gestation and their child's externalizing behavior at 18 months of age by means of standardized questionnaires. Data were analyzed using logistic regression, with scores of externalizing behavior above the 88.6th percentile as the dependent variable and self-reported smoking as the independent variable. We examined the child's sex as a possible moderator. Results: We documented a threshold effect of smoking 10 cigarettes or more per day during pregnancy on subsequent externalizing behaviors among 18-month-old children, even after adjusting for relevant confounders (odds ratio 1.32, 95% confidence interval 1.03-1.70). The child's sex did not moderate these effects (odds ratio 0.98, 95% confidence interval 0.83-1.16). Conclusions: Maternal smoking during pregnancy increases offspring's subsequent risk for externalizing behavior problems at 18 months of age. The pattern of risk does not differ between boys and girls. Our findings suggest a population attributable risk of 17.5% (i.e., the proportion of externalizing cases that could potentially be avoided if prenatal smoking was eliminated or reduced to fewer than 10 cigarettes per day). J. Am. Acad. Child. Adolesc. Psychiatry, 2009;48(3):283 289.