A ureteropelvic junction obstruction (UPJO) is a congenital narrowing or obstruction where the ureter meets the kidney. These obstructions develop prenatally as the kidney is forming. Studies confirm that it is more common in males and on the left side. However, both kidneys may be involved.

This narrowing will not allow urine to drain from the kidney, causing hydronephrosis (kidney swelling). With the use of pre-natal ultrasounds, hydronephrosis is often diagnosed as early as 15 weeks of fetal development. UPJO is the most common cause for pediatric hydronephrosis. Early detection allows for evaluation and treatment soon after birth.