Typical kidney and a kidney with hydronephrosis in an infant

Hydronephrosis happens when urine builds up in a kidney. Often, a blockage called an obstruction in the upper part of the urinary tract causes the buildup. Kidney swelling can result. This causes the part of the kidney called the renal pelvis to bulge, or become distended. It could lead to kidney scarring, and the kidney may not work as well as it should.