Newborn with left hydronephrosis on prenatal US

US of ureteropelvic junction obstruction
Sagittal US of the right kidney (above) shows a single renal collecting system with no evidence of hydronephrosis. Sagittal US of the left kidney (below) show a single renal collecting system that has a moderate amount of hydronephrosis. The left renal pelvis is also moderately dilated. There was no left hydroureter.

The diagnosis was left ureteropelvic junction obstruction.