Smoking during pregnancy and hyperactivity-inattention in the offspring - Comparing results from three Nordic cohorts

Abstract

Background: Prenatal exposure to smoking has been associated with Attention Deficit Hyperactivity Disorder (ADHD) in a number of epidemiological studies. However, mothers with the ADHD phenotype may 'treat' their problem by smoking and therefore be more likely to smoke even in a society where smoking is not acceptable. This will cause genetic confounding if ADHD has a heritable component, especially in populations with low prevalence rates of smoking since this reason for smoking is expected to be proportionally more frequent in a population with few 'normal' smokers. We compared the association in cohorts with different smoking frequencies. Methods: A total of 20 936 women with singleton pregnancies were identified within three population-based pregnancy cohorts in Northern Finland (1985-1986) and in Denmark (1984-1987 and 1989-1991). We collected self-reported data on their pre-pregnancy and pregnancy smoking habits and followed the children to school age where teachers and parents rated hyperactivity and inattention symptoms. Results: Children, whose mothers smoked during pregnancy, had an increased prevalence of a high hyperactivity-inattention score compared with children of nonsmokers in each of the cohorts after adjustment for confounders but we found no statistical significant difference between the associations across the cohorts. Conclusion: The estimated association was not strongest in the population with the fewest smokers which does not support the hypothesis that the association is entirely due to genetic confounding. © Published by Oxford University Press on behalf of the International Epidemiological Association © The Author 2008; all rights reserved.