Smoking was weakly associated with the risk of placenta previa (table 4). When adjusted for confounding due to marital status, maternal age, parity, chronic hypertension, preeclampsia, and alcohol use, the relative risk for placenta previa was 1.36 (95 percent CI 1.04-1.79). No dose-response gradient was observed in relation to the number of cigarettes smoked per day. Nevertheless, women who smoked 16-20 cigarettes per day were at 80 percent (95 percent CI 1.06-3.07) increased risk of placenta previa in comparison with nonsmokers. The risk of placenta previa was slightly elevated for women who drank alcohol during pregnancy (RR = 1.26, 95 percent CI 0.95-1.69), after adjustment for confounders.