Hydronephrosis of pregnancy: ultrasonographic study and classification of asymptomatic women.

Ultrasonographic examination of the upper urinary tract in 109 asymptomatic pregnant women was performed to determine the degree of dilatation, the time of onset, and the range of "physiologic" changes detectable by this method. Classification is established and prevalence and degree of dilatation are categorized by trimesters. The onset of dilatation is seen earlier than previously reported and the degree of obstruction is quite variable for the asymptomatic patient. The right side predominates, as expected. The finding of mild dilatation in a symptomatic patient would prompt a search for causes other than obstruction.