Hydronephrosis frequently occurs during pregnancy. The etiology and significance of this phenomenon are not clearly understood. Further, little information regarding changes in the kidneys during pregnancy and early postpartum is available. This study uses real-time ultrasound in order to investigate whether or not there is a relationship between the degree of calyceal dilatation, kidney size, and creatinine clearance value. Two hundred and eighty-six patients, including 175 pregnant and Ill puerperal individuals, were examined. The overall incidence of hydronephrosis was 67% in the pregnant and 39% in the puerperal women. Moderately and severely affected kidneys were significantly larger than the mildly affected and nonaffected kidneys (p < 0.05, p < 0.001). The creatinine clearance value was lower in patients with severe hydronephrosis than in those with mild and moderate hydronephroses and in the nonaffected patients, albeit with no statistical significance.