Birth trauma is the most important etiological factor in the genesis of stress urinary incontinence in women (SUI). There is a high incidence of SUI during pregnancy, and after delivery SUI persists in a small percentage of women (2-3%). Almost all studies on perineal muscle function reveal decreasing intravaginal pressures in the days after delivery which rarely return to predelivery levels. A few urodynamics studies have demonstrated reduced urethral closure pressures and functional length after vaginal delivery, but the importance of such findings in the genesis of SUI is controversial. Better consensus has been found when pelvic floor neurophysiology was carried out: there is electromyographic evidence of a denervation-reinnervation pattern in the striated urethral sphincter muscle and occasionally prolonged pudendal conduction times when the pudendal nerves are directly stimulated. Histomorphologic studies of the pelvic floor have demonstrated that, in some women, abnormal collagen types are responsible for vaginal prolapse and accompanying SUI. Finally, the great importance of perineal reeducation by electromyostimulation and biofeedback in patients with traumatic pelvic floor pathology may be emphasized, but the importance of its role in the prevention of late SUI development remains to be established by more prospective studies.