Maternal smoking in pregnancy and asthma in preschool children: a pooled analysis of eight birth cohorts.

RATIONALE Although epidemiological studies suggest that exposure to maternal smoking during fetal and early life increases the risk of childhood wheezing and asthma, previous studies were not able to differentiate the effects of prenatal from postnatal exposure. OBJECTIVES To assess the effect of exposure to maternal smoking only during pregnancy on wheeze and asthma among preschool-age children. METHODS A pooled analysis was performed based on individual participant data from eight European birth cohorts. Cohort-specific effects of maternal smoking during pregnancy, but not during the first year, on wheeze and asthma at 4 to 6 years of age were estimated using logistic regression and then combined using a random effects model. Adjustments were made for sex, parental education, parental asthma, birth weight, and siblings. MEASUREMENTS AND MAIN RESULTS Among the 21,600 children included in the analysis, 735 children (3.4%) were exposed to maternal smoking exclusively during pregnancy but not in the first year after birth. In the pooled analysis, maternal smoking only during pregnancy was associated with wheeze and asthma at 4 to 6 years of age, with adjusted odds ratios of 1.39 (95% confidence interval, 1.08-1.77) and 1.65 (95% confidence interval, 1.18-2.31), respectively. The likelihood to develop wheeze and asthma increased statistically significantly in a linear dose-dependent manner in relation to maternal daily cigarette consumption during the first trimester of pregnancy. CONCLUSIONS Maternal smoking during pregnancy appears to increase the risk of wheeze and asthma among children who are not exposed to maternal smoking after birth.