The current study evaluated the level of concordance for maternal and paternal smoking during pregnancy and examined whether maternal and paternal smoking during pregnancy is associated with elevated inattention and hyperactivity/impulsivity scores among preschool children as assessed by teacher and parent reports. Further, we examined whether maternal and paternal smoking during pregnancy is associated with an increased risk for ADHD, ODD or comorbid ADHD and ODD. Our data are consistent with, and expand results from, prior studies, providing 3 main findings. First, mothers were over 8 times more likely to have smoked during pregnancy if her partner smoked during her pregnancy. Second, negative effects of maternal, but not paternal, were documented with respect to ADHD symptoms. Third, children of smoking, as compared with children of nonsmoking, mothers were at an increased risk for developing comorbid ADHD and ODD, and ADHD alone, but not ODD alone. Father’s smoking during pregnancy had no notable influence on diagnostic outcomes.
Potential damage to the fetus attributed to prenatal exposure to smoking in utero is widely recognized. Pregnant mothers are always advised by their obstetric care providers, and many times informally by friends and relatives, to give up smoking during pregnancy. Despite that, pregnancy has not proved to be an effective deterrent to smoking (Haug et al., 1994
; Ockene, 1993
). Prior research reported an association between smoking during pregnancy and maternal psychopathology such as depression, antisocial personality disorders and addiction, and maternal ADHD. To the extent that ADHD has been associated with smoking, this could be one of the compelling reasons why some mothers with genetic susceptibility for ADHD could not give up smoking during pregnancy despite the well-known risks to the fetus (Kollins et al., 2005
). In our sample, there was a significant difference on maternal ADHD symptoms between mothers who smoked and mothers who did not (47.97 vs. 45.59, p
= 0.02), although few mothers in each group had clinically significant ADHD symptomatology. Therefore, it is possible that the offspring of smoking mothers compared with non-smoking mothers have higher risk for ADHD because of both environmental and familial influences. However, to understand the effect of smoking during pregnancy as an environmental risk, the effect of both mothers and fathers ADHD symptom ratings, as derived from the Conners’ Adult ADHD Rating Scale (Conners et al., 1999
) were adjusted for in all of our analysis. Although paternal smoking was associated with an increased risk for maternal smoking during pregnancy (OR = 8.6, p
< 0.0001), maternal, but not paternal, smoking was associated with elevated inattention and hyperactivity/impulsivity scores, as well as ADHD diagnosis and comorbid ODD in children. Taken together, associations between maternal smoking during pregnancy and elevated inattention and hyperactivity scores and the risk for ADHD (both comorbid with ODD and ADHD alone) appears to be due to the exposure to cigarette smoking in utero, independent of a familiar risk for ADHD.
Our study has methodological strengths. First, using multiple informants bring together ratings of behaviors across home and preschool environments, and offsets potential rating biases associated with each type of informant. Despite a tendency to treat teacher reports, rather than the parent reports, as the gold standard in developmental psychopathology research, the literature also suggests that children behave differently at home and at school (Woo and Rey, 2005
). To maximize multiple informant data, we made novel use of an analytic strategy (GEE) that reduces informant bias by taking account of correlations between multiple informant reports. The analytic techniques combining mother and teacher reports produce more reliable measures, and lead to more precise estimates of the associations between parental smoking (the risk factor) and child behavioral problems (Richters and Pellegrini, 1989
; Chilcoat and Breslau, 1997
). Second, we had both dimensional measures of behavioral problems related to ADHD and DSM-based ADHD and ODD diagnoses ascertained by semi-structured psychiatric interviews. Final diagnoses were assigned to each child at the diagnostic consensus meeting.
Our study also has limitations. First, smoking is based on retrospective self-report, which is susceptible to threats to reliability and validity. Given the stigma against smoking during pregnancy, it is possible that smoking mothers, more so than smoking fathers, under-reported their smoking during pregnancy. Smoking mothers with a child who has behavioral problems may be more likely to look for reasons why their child has problems. This may cause them to report smoking during pregnancy more accurately than smoking mothers who had a child without behavioral problems. We incorporated both the mothers’ and teachers’ reports of inattention and hyperactivity/impulsivity in children to minimize potential reporter bias; however, prospective data on smoking or a measure of serum cotinine would have strengthened our findings greatly. Second, we do not have family history of ADHD, although we have measures of hyperactivity and inattention from the parents, and these effects were adjusted in all analyses. However, as the risk of smoking is known to be higher among those with ADHD than those without, it is possible that the associations between maternal smoking during pregnancy and the child’s impairment in attention and increased risk for ADHD may be due to genetically correlated environmental risk (i.e., exposure to maternal smoking in utero which is higher among mothers with ADHD than mothers without ADHD). As we found no association between paternal smoking during pregnancy and child impairment and increased risk, we suggest that significant associations between elevated inattention scores and the risk for ADHD may be due to the fetus being exposed to an environmental toxin such as smoking. Ascertaining parents’ lifetime ADHD status will significantly contribute to further clarification of this issue. Because the study is ongoing, we plan to collect a diagnosis of ADHD from parents and the clarification on parental ADHD status will inform our future studies.
Despite these limitations, the current study shows that maternal, but not paternal, smoking during pregnancy is associated with significantly higher inattention, hyperactivity, and total ADHD scores and with an elevated risk for ADHD (both comorbid with ODD and ADHD alone) among preschool children. Maternal smoking was not associated with ODD alone, but ODD is highly associated with ADHD (OR = 11.4, 95% CI = 6.0–21.6, p < 0.0001). It is therefore possible that maternal smoking is associated with ADHD, and that we observed the association between maternal smoking and ODD through its high correlation with ADHD. It is also possible that maternal smoking is associated with an increased risk for ADHD, and that, in turn, increases the risk for subsequent ODD. However, we need to be cautious about concluding that maternal smoking is primarily associated with an increased risk for ADHD, because a nonsignificant association between maternal smoking and ODD alone could be explained by lack of sufficient statistical power to detect the existing difference because of the small number of cases with only ODD. The current study has a primary focus on ADHD and therefore the sample size by itself does not allow us to test a role of maternal smoking on both ADHD and ODD. With a larger sample size, future studies may inform whether maternal smoking is a risk factor for both ADHD and ODD independently.
We had hypothesized that children of parents who both smoked would have the highest risk for behavioral problems. However, our data suggested that paternal smoking had no notable influence in behavioral problems: children of both parents who smoked and mother-only smoking had similar behavioral characteristics, and children in which neither parent smoked or only the father smoked also had similar behavioral characteristics. Moreover, parental smoking was not associated with any child outcomes (comorbid ADHD and ODD, ADHD alone, and ODD alone). Although we do not have a direct measure of postnatal smoking among parents, considering the addictive nature of smoking, it is likely that fathers who smoked during his partner’s pregnancy were likely to continue smoking after the child was born. Therefore, while definitive understanding needs to wait for future studies and replications, we conclude that prenatal fetal exposure to smoking has a time-specific high toxic valence on offspring’s optimal development.
Taken together, these findings suggest that maternal smoking during pregnancy represents a serious behavioral risk factor for ADHD in their offspring. As their partner’s smoking had a striking influence on the risk of women indulging in the same risk behaviors during pregnancy, intervention for both mothers and fathers might be an effective means for preventing subsequent ADHD-related behavior, symptoms, and diagnosis in children. Developing a refined understanding is urgently needed to increase mothers’ awareness of the risk of smoking during pregnancy to mitigate the long-term human and economic costs to children, their families, and society.