Intrauterine exposure to tobacco and risk of medically indicated and spontaneous preterm birth.

We investigated the association between prenatal smoking and the occurrence of medically indicated and spontaneous preterm delivery (<37 weeks). We performed a retrospective cohort study of singleton live births in the state of Missouri (n = 1,219,159) using maternally linked cohort data files covering the period 1989 to 2005. The main outcomes of interest were spontaneous and medically indicated preterm and very preterm birth. Logistic regression models were used to generate adjusted odds ratios and their 95% confidence intervals. There were 132,246 (10.8%) infants born preterm in the study population, of which 106,410 (80.5%) were classifiable as spontaneous preterm births and 25,836 (19.5%) were medically indicated preterm deliveries. We found elevated risks for both medically indicated and spontaneous preterm birth associated with maternal cigarette smoking during pregnancy. This heightened risk was particularly evident for medically indicated preterm birth (adjusted odds ratio [95% confidence interval] = 1.48 [1.41 to 1.55]). Women who smoke during pregnancy are at increased risk for preterm birth, and especially for medically indicated preterm delivery.