When the number of cigarettes smoked daily was
entered into the model as a continuous variable with a
cubic spline transformation, the risk of abruption increased up to a level of 10 cigarettes per day, and
thereafter remained constant at a relative risk of 2.1
(figure 1). However, the pattern was quite different for
placenta previa and uterine bleeding of unknown etiology: Compared with nonsmoking, the adjusted relative risk for number of cigarettes smoked per day was
almost flat around the null value.