Objective: To compare, in an unselected population of nulliparous pregnant women, the postnatal effect of prenatal supervised pelvic floor muscle training compared with written instructions on postpartum urinary incontinence (UI). Methods: In a randomized controlled trial in two parallel groups, 282 women were recruited from five university teaching hospitals in France and randomized during the second trimester of pregnancy. The physiotherapy group received prenatal individually supervised exercises. Both groups received written instructions about how to perform exercises at home. Women were blindly assessed at baseline, end of pregnancy, and 2 and 12 months postpartum. The primary outcome measured was UI severity, assessed with an International Consultation on Incontinence Questionnaire Urinary Incontinence- Short Form score (range 0-21; 1-5 is slight UI) at 12 months postpartum, other outcomes were UI prevalence and pelvic floor troubles assessed using self-administered questionnaires. In order to give a 1-point difference in UI severity score, we needed 91 women in each group (sd=2.4, =0.05, β=0.20 and bilateral analysis). Results: Between February 2008 and June 2010, 140 women were randomized in the physiotherapy group and 142 in the control group. No difference was observed between the two groups in UI severity, prevalence, or pelvic floor troubles at baseline, end of pregnancy, and at 2 and 12 months postpartum. At 12 months postpartum, the primary outcome was available for 190 women (67.4%); mean UI severity was 1.9 in the physiotherapy group versus 2.1 in the control group (p=0.38). Conclusion: Prenatal supervised pelvic floor training was not superior to written instructions in reducing postnatal UI.