OBJECTIVE To estimate the incidence and relative risk of postpartum urinary incontinence in primiparas. METHODS Over 3 years (1996-1998), healthy nulliparas with no significant history of urinary tract abnormalities were prospectively enrolled. Power analysis indicated that 452 women must complete the study. Participants completed a questionnaire about urinary, fecal, and flatal incontinence. At 6 weeks' and 6 months' postpartum, the same questionnaire was mailed to participants. Statistical analysis consisted of chi-square and Fisher exact tests for nominal variables and analysis of variance and Kruskal-Wallis tests for interval variables. Logistic regression tested independence of variables. RESULTS A total of 690 primiparas were enrolled in the study and 595 delivered at our hospital: 147 (25%) cesareans, 333 (56%) spontaneous vaginal deliveries, and 115 (19%) instrumental vaginal deliveries. Median birth weight was 3489 g. Urinary incontinence rate at 6 months was 26%. Vaginal delivery was associated with a higher incidence of urinary incontinence (relative risk, 2.8) compared with cesarean. Forceps delivery increased the risk of urinary incontinence (relative risk, 1.5) compared with spontaneous vaginal delivery. There was no significant difference between cesareans done before and during labor. None of the obstetric risk factors were independently significant. CONCLUSIONS Cesarean delivery at any stage of labor reduces postpartum urinary incontinence. With multivariable analysis, obstetric risk factors are not significant.