INTRODUCTION: Female urinary incontinence (UI) is a frequent affection that generates handicap and expenses. There is a link between UI and pregnancy; onset of UI during pregnancy is a risk factor for permanent UI. Postnatal pelvic floor exercise has shown efficacy to improve postnatal UI. However, it remains uncertain if benefits last more than few months. Publication of our rationale for prenatal pelvic floor exercise is an opportunity to expose our pre-specified hypotheses and help health professionals' awareness. OBJECTIVES: The purpose of PreNatal Pelvic floor Prevention (3PN) is to compare the effects of prenatal pelvic floor exercise versus sole written instructions on UI one year after delivery. METHODS AND POPULATION: It is a multicenter, randomized, single blind study. Main inclusion criteria are first, single and non-complicated pregnancy over 18 years. Women randomized in pelvic floor exercise group will undergo eight sessions with a physiotherapist between six and eight months of pregnancy. Our principal criterion is UI score (International Consultation on Incontinence Questionnaire Short Form [ICIQ-SF]) one year after delivery. We plan to include 280 pregnant women in five centers over a 12-month screening period to show a one-point difference on UI score. ETHIC AND FINANCING: The study was approved by the IRB Comit?e protection des personnes Sud-Ouest et Outre-Mer. It was registered by French Health Products Safety Agency (AFSSAPS) and Clinical Trials.gov. It is supported by the French Ministry of Health through the 2007 Hospital Plan for Clinical Research (PHRC). PERSPECTIVES: We plan to assess if prenatal pelvic floor exercise reduces postnatal medical consultations or physiotherapy sessions.