Pregnancy hydronephrosis--a longitudinal ultrasonic evaluation.

The progression of physiological hydronephrosis through the 3 trimesters of pregnancy was assessed by ultrasonic measurement of calyceal diameters in 56 asymptomatic patients in a longitudinal study. A grading system basing on the maximum calyceal diameter was used; 89% of the patients showed at least mild dilatation before delivery. There was a preponderance of the right side being affected. In 6 patients (10.7%), severe dilatation was demonstrable and in all of them its onset appeared to be in the second trimester. The likelihood of progression to moderate or severe dilatation when only mild dilatation is demonstrated in the second trimester is low. Furthermore, it is not common for moderate dilatation in the second trimester to progress to severe dilatation in the third trimester. These findings appear to support the mechanical theory in the genesis of pregnancy hydronephrosis. Rapid regression of calyceal dilatation is not apparent in the first 5 days after delivery.