MATERNAL CIGARETTE-SMOKING AND CHILD PSYCHIATRIC MORBIDITY - A LONGITUDINAL-STUDY

Objective. Previous studies have linked maternal smoking during pregna ncy with behavioral disturbance in children. However, additional evide nce is needed to address the causality of the relationship. The presen t study analyses result from an Australian cohort of 5342 5-year-old c hildren whose mothers were recruited early in pregnancy. Methods. Smok ing history was gathered for prepregnancy, first clinic visit (FCV), l ate pregnancy, and when the child was 6 months and 5 years of age. Beh avior problems at the age of 5 were assessed using a modified Child Be havior Check List (CBCL) shown to have high agreement with the complet e CBCL. This resulted in the formation of three scales: internalizing; social, attentional and thought; and externalizing behavior problems, which were then dichotimized at the 90th percentile in each case. Log istic regression was used to model these outcomes as a function of mat ernal smoking at five time points during which it was assessed. A seri es of models explored the effect of additional adjustment for confound ing. The predictors of attrition (29.5%) throughout the cohort were al so identified by multivariate modeling. Results. The final analysis wa s carried out on a cohort of mother-child pairs for whom data and chil d behavior outcomes were complete. The mean age of children was 5 year s, 6 months with a range from 4 to 6 years. The mean age of mothers at the time of birth of the child was 25 years, with a range from 13 to 47 years. Mothers lost to follow-up were more likely to be younger, si ngle, and less well-educated than those who continued participation, a lthough maternal smoking was not an independent determinant. Unadjuste d analyses showed strong associations between externalizing child beha vior and maternal smoking during pregnancy and at the 5-year follow-up , with relative risks (RRs) up to 2.6 for children of women smoking at least 20 cigarettes per day at the first antenatal clinic visit. A cl ear dose-response relationship existed in most relationships with high er levels of smoking being associated with higher rates of externalizi ng behavior problems. Weaker relationships occurred for internalizing behavior and social, attentional and thought behavior problems. Multiv ariate analysis of the timing in more detail that. the association bet ween maternal smoking and child behavior problems persisted, although the evidence for dose-response diminished. Moreover, it was primarily associated with smoking as determined by questions asked at the FCV (R R = 1.52, 2.03, 2.16) for 1 to 9, 10 to 19, and greater than or equal to 20 cigarettes per day, respectively, compared with nonsmoking and s econdarily by smoking determined at the 5-year follow-up (RR = 1.52, 1 .87, 1.29) for 1 to 9, 10 to 19, and 20 cigarettes per day respectivel y, compared with nonsmoking. This association appeared to be independe nt of a wide range of possible confounders such as maternal age, educa tion, social class, marital status and mental health, gestation at FCV , complications during pregnancy, the child's sex, gestational age at birth, and age at last follow-up. Adjustments were also made for the m other's employment since birth, family structure, and maternal mental health at the time of the CBCL assessment. Associations between extern alizing behavior problems and maternal smoking at other times, and tho se between other behavioral problems examined and maternal smoking wer e not significant. Conclusion. Although previous studies have found ev idence for an association between maternal smoking and child behavior problems, the strength of this study lies in its size, its detailed an d consistent measurement of maternal smoking, and its ability to contr ol for many social and biological factors linked to maternal smoking a nd child behavior. The statistical evidence for a causal relationship between maternal smoking as measured at the first antenatal clinic vis it and the development of externalizing behavior in children is strong ly suggestive, because of the specificity of effect and timing, and th e adjustment for a comprehensive range of other risk factors. An alter native explanation that mothers who smoke might be inclined to report behavior problems differently was considered unlikely because the effe ct was specific to externalizing behavior and the major exposure occur red 5 years before the reported behavior. Additionally, the existence of a factor (beyond those already included in the analysis) that might be related to both behavior problems and any propensity to misreport smoking seems sufficiently implausible to rule out misclassification o f smoking status as another alternative explanation for the findings. The timing of the relationship between smoking in pregnancy and extern alizing behavior problems, and its independence of small for gestation al age status at birth, suggests that placental insufficiency and oxyg en deprivation are not important in the pathogenesis. It is possible t hat nicotine in the fetal brain derived from maternal smoking may alte r gene expression and the nature and function of the nicotine receptor s developing in early pregnancy. The presence of a separate, although slightly weaker, association between concurrent maternal smoking and e xternalizing child behavior suggests both biological and social explan ations, including a direct psychopharmocologic effect of nicotine thro ugh passive smoking. Assuming the relationship to be causal, it is est imated that maternal smoking in early pregnancy may account for 25% of externalizing (aggressive) behavior while maternal smoking when the c hild is 5 years old may account for an additional 16%. These findings provide further support for antismoking programs in pregnancy and in y oung family settings.