Three hundred five primiparas were interviewed repeatedly about stress incontinence before and during pregnancy and after delivery. Eleven (4%) had stress incontinence before pregnancy and 98 (32%) during pregnancy, whereas 21 (7%) developed it after delivery. According to the International Continence Society definition, the corresponding frequencies were one (0.3%), three (1%), and one (0.3%), respectively. Obstetric factors such as length of the second stage of labor, head circumference, birth weight, and episiotomy seemed to be associated with, whereas cesarean delivery seemed to protect against, the development of stress incontinence after delivery. Three months after delivery, the statistically significant influence of the obstetric factors had vanished, as stress incontinence had disappeared in most women. However, 1 year after delivery eight of 292 women (3%) had stress incontinence, three with onset during pregnancy and five with onset after delivery. Three of these eight had stress incontinence according to International Continence Society criteria; four women wanted treatment. The symptom of stress incontinence occurs as a natural consequence of pregnancy and delivery and generally resolves in the puerperium. However, pregnancy and delivery carry a small risk (1% or less) of initiating persistent stress incontinence. The importance of various obstetric factors seems transient and their etiologic role remains unclear.