Maternal smoking is associated with an increased prevalence of respiratory morbidity in children. It had been widely assumed in the past that this effect was the result of postnatal environment tobacco smoke exposure (passive smoking). There is mounting evidence, based on studies in humans and in animal models to suggest that maternal smoking during pregnancy adversely affects fetal lung development. The pathogenesis for this lesion is unclear and it is not known if the insult is the same in the human and the animal model.