Smoking during pregnancy has been consistently linked with externalizing (behaviour) problems in children, particularly in males. It has been associated with parent-reported conduct problems, arrest history from national crime registries, contact with police, oppositional defiant disorder and attention-deficit/hyperactivity disorder. However, there are obvious ethical difficulties with controlled trials exposing fetuses to maternal smoking during pregnancy and it could be the case that the association between smoking during pregnancy and behaviour problems is due to other environmental factors that lead pregnant women to smoke and their children to become badly behaved. Previous studies have attempted to take these factors into account but a study of 4,886 women in the U.S. went further by comparing the behaviour of their children who had grown up in the same family but who had been exposed to different levels of nicotine in the womb. When these children were compared there was no significant difference in conduct problems or oppositional defiant problems between the children who had and had not been exposed to nicotine in utero. There was a small link between maternal smoking and ADHD but the researchers concluded that 'genetically informed analyses suggest that unmeasured environmental variables influencing both smoking during pregnancy and offspring externalizing behaviours account for the previously observed associations'.
D'Onofrio, Brian M. ... [et al] - Smoking during pregnancy and offspring externalizing problems: an exploration of genetic and environmental confounds Development and psychopathology (2008), 20: 121-137