Relationship of maternal smoking during pregnancy to placenta previa.

Although maternal smoking during pregnancy is reportedly associated with placenta previa, its etiology is unknown. We examined this association by using North Carolina vital records of 1988 and 1989. Among women who smoked 0, 1-9, 10-19, and > or = 20 cigarettes per day during pregnancy, the prevalence of placenta previa at delivery was 3.8, 5.7, 6.3, and 6.7 per 1,000 singleton live births, respectively, in the entire population. However, after the potential confounders (maternal age, race, gravidity, parity, previous pregnancy terminations, previous cesarean section, and gestational age) were controlled by individual matching, the adjusted overall odds ratio (OR) was 1.29 (95% confidence interval [CI] 1.05, 1.58) with slight dose-response gradients. Our study indicates that, although maternal smoking during pregnancy might affect placenta previa, the magnitude is substantially smaller than previously reported. This association may be attributable to other factors, such as detection bias.