Objectives: We examined the relationships among maternal smoking in pregnancy, fetal development, and the risk of asthma in childhood.
Methods: We conducted a population-based cohort study, where all 58 841 singleton births were followed for 7 years using nationwide registries.
Results: Maternal smoking increased the risk of asthma (adjusted odds ratio = 1.35; 95% confidence interval = 1.13, 1.62 for high exposure). Low birthweight and preterm delivery increased the risk of asthma at the age of 7, whereas being small for gestational age did not.
Conclusions: Maternal smoking in pregnancy increases the risk of asthma during the first 7 years of life, and only a small fraction of the effect seems to be mediated through fetal growth.