Thirty-four pregnant primigravidae underwent serial real-time nephrosonography during their pregnancy and for three months post partum. The study confirms the asymmetrical effect of pregnancy on upper urinary tract dilatation as well as the incidence of changes as reported by others. Unlike previous published work, the serial nature of this study makes it the first to describe in detail the degree of change which occurs with advancing gestation and allows the study of the dynamics of pregnancy hydronephrosis. It demonstrates that twenty weeks of gestation marks the threshold for the development of significant dilatation and shows a long-term effect of pregnancy on the distensibility of the upper urinary tract.