vations, while Williams et al. (14) did not observe any dose-response pattern. Similarly, in an analyses of placental abruption, Williams et al. (4) observed an odds ratio of 1.4 (95 percent CI 0.8-2.2) for smoking 1-9 cigarettes per day and 1.5 (95 percent CI 0.8-2.8) for 10-19 cigarettes per day. In comparable analyses, we observed relative risks of 1.3 (95 percent CI 0.9 2.0) and 1.4 (95 percent CI 1.0-1.8) for placenta previa and 1.9 (95 percent CI 1.5-2.4) and 2.2 (95 percent CI 1.9-2.6) for placental abruption among women who smoked 1-10 and 10 cigarettes per day, respectively. Both analyses indicate the presence of a modest dose-response gradient, although this pattern vanishes when number of cigarettes smoked per day is stratified more finely. relating Several potential mechanisms