Associations of prenatal maternal smoking with offspring hyperactivity: causal or confounded?

Background The relationship between prenatal tobacco exposure and hyperactivity remains controversial. To mitigate limitations of prior studies, we used a strategy involving comparison of maternal and paternal smoking reports in a historical sample where smoking during pregnancy was common. Method Data were drawn from a longitudinally followed subsample of the Child Health and Development Study (n=1752), a population-based pregnancy cohort ascertained in 1961-1963 in California. Maternal prenatal smoking was common (33.4%). Maternal and paternal smoking patterns were assessed at three time points by mother report. Hyperactivity was assessed at the mean of age of 10 years based on mother report to a personality inventory. Results Unadjusted, maternal smoking during pregnancy was associated with offspring hyperactivity [beta=0.22, 95% confidence interval (CI) 0.11-0.33] and, to a similar degree, when the father smoked (beta=0.18, 95% CI 0.07-0.30). After adjustment, maternal smoking remained robustly predictive of offspring hyperactivity (beta=0.25, 95% CI 0.09-0.40) but father smoking was not (beta=0.02, 95% CI -0.20 to 0.24). When examined among the pairs matched on propensity score, mother smoking was robustly related to offspring hyperactivity whether the father smoked (beta=0.26, 95% CI 0.03-0.49) or did not smoke (beta=0.30, 95% CI 0.04-0.57). By number of cigarettes, associations with hyperactivity were present for 10-19 and 20+ cigarettes per day among mothers. Conclusions In a pregnancy cohort recruited in a time period in which smoking during pregnancy was common, we document associations between prenatal smoking exposure and offspring hyperactivity. Novel approaches to inferring causality continue to be necessary in describing the potential adverse consequences of prenatal smoking exposure later in life.