Maternal Smoking, Birthweight, and Childhood Behavioral Problems in North America

INTRODUCTION: Maternal smoking during pregnancy has been associated with behavioral problems in offspring.  However, these observed associations have raised questions about direct and indirect effects.  In particular, exposures at birth and during childhood have been suspected to account for such associations.  Maternal smoking decreases birth weight, birth weight is associated with childhood behavioral problems, and smoking and birth weight interact in their association with childhood behavioral problems.

METHODS: In this study, we measured the direct and indirect effects of maternal smoking on behavioral outcomes at age 5 to 12 years, while considering birthweight as a potential mediator.   The study population included 560 mothers who had served as controls in a North American case-control study of a craniofacial birth defect and whose child was assessed for behavior problems.  Child’s internalizing behavior problems (e.g., anxiety, depression) and externalizing behaviors (e.g., oppositional behavior) were measured by mother’s report (Child Behavior Checklist) and teacher’s report (Teacher Report Form).  Mean differences in scores were compared between the children of mothers who smoked during pregnancy (n=89) and non-smokers (n=471) with linear regression models, after adjustment for maternal age, race/ethnicity, education, income, pre-pregnancy body mass index, multivitamin use, and region.   

RESULTS:  According to both maternal and teacher reports, adjusted mean internalizing behavior scores were similar for children of smokers and non-smokers.  Externalizing scores were higher (worse) for children of smokers: the adjusted mean difference for mother report was 2.16; 95%CI: -0.21, 4.54 and for teacher report was 2.69; 95%CI 0.55, 4.83.  When terms for low birthweight (<2500 g) and its interaction with smoking were included in the model, the indirect effects of maternal smoking on externalizing scores through low birthweight were small and mediation was <1%.

CONCLUSIONS:  The association of maternal smoking and externalizing behavior problems in offspring does not appear to operate through low birthweight.