Hydronephrosis in pregnancy and its etiology

Using ultrasonography, the cause and incidence of hydronephrosis were investigated in humans and rabbits and the following results were obtained. The degree of hydronephrosis was classified into grade I (calyceal diameter; up to 5 mm) to grade V (calyceal diameter; 21 mm or more). In normal non-pregnant women, the calyceal diameter was within grade I. The hydronephrosis was found in 45.5-87.0% of pregnant women in the right kidney and in 29.1-51.0% in the left. Throughout pregnancy, the incidence was significantly higher in the right than in the left. On the 6th puerperal day, the incidence of hydronephrosis was lowered, but the diameter did not return to the non-pregnant level. Comparison of women who practised swimming during pregnancy and those who did not reveal the incidence of hydronephrosis was lower in the swimming group than in the non-swimming. In rabbits, calyceal dilatation increased as gestation advanced and hydronephrosis was found in progesterone-treated ovariectomized rabbits. It was concluded that the hydronephrosis in the 1st trimester was due to increased steroid hormones, but also that, in the 2nd trimester, the combined effect of hormones and mechanical obstruction was responsible for its occurrence.