Maternal smoking and child psychological problems: Disentangling causal and noncausal effects

OBJECTIVE: To explore associations of maternal prenatal smoking and child psychological problems and determine the role of causal intrauterine mechanisms. PATIENTS AND METHODS: Maternal smoking and child psychological problems were explored in 2 birth cohorts in Pelotas, Brazil (n = 509, random subsample), and the Avon Longitudinal Study of Parents and Children (ALSPAC) in Britain (n = 6735). Four approaches for exploring causal mechanisms were applied: (1) crosspopulation comparisons between a high-income and a middleincome country; (2) multiple adjustment for socioeconomic and parental psychological factors; (3) maternal-paternal comparisons as a test of putative intrauterine effects; and (4) searching for specific effects on different behavioral subscales. RESULTS: Socioeconomic patterning of maternal prenatal smoking was stronger in the ALSPAC compared with the Pelotas cohort. Despite this difference in a key confounder, consistency in observed associations was found between these cohorts. In both cohorts, unadjusted maternal smoking was associated with greater offspring hyperactivity, conduct/externalizing problems, and peer problems but not with emotional/internalizing problems. After adjusting for confounders and paternal prenatal smoking, only the association with conduct/externalizing problems persisted in both cohorts (conduct problems in the ALSPAC cohort, odds ratio [OR]: 1.24 [95% confidence interval (CI): 1.07-1.46], P = .005; externalizing problems in the Pelotas cohort, OR: 1.82 [95% CI: 1.19-2.78], P = .005; ORs reflect ordinal odds ratios of maternal smokers having offspring with higher scores). Maternal smoking associations were stronger than paternal smoking associations, although statistical evidence that these associations differed was weak in 1 cohort. CONCLUSION: Evidence from 4 approaches suggests a possible intrauterine effect of maternal smoking on offspring conduct/externalizing problems.