In a population sample, the period prevalence of urinary incontinence (UI) during pregnancy was found to be 19.9% and 24.1% among 352 nulliparous and 290 primiparous women, respectively. The first UI episode ever was experienced by 16.7% and 7.0% during the two last trimesters of the first and second pregnancies, respectively. None of the pregnancy-specific risk factors, such as emesis and birthweight, was significantly associated with UI during pregnancy. Previous UI was a significant risk factor for period prevalent UI during pregnancy, explaining 34% and 83% of pregnancy UI for the nulliparous and the primiparous, respectively. The present data suggest pregnancy UI not to be provoked by the mere onset of pregnancy, but by increasing hormonal concentrations or local tissue changes caused by hormones, whereas there was no support for a theory based on increasing pressure on the bladder caused by the weight of the fetus.