Also called: Unilateral Hydronephrosis; Bilateral Hydronephrosis; Urinary Obstruction; Swollen Kidneys

Hydronephrosis (hi-dro-nih-FRO-sis) is when one or both kidneys become swollen because the flow of urine (pee) is backed up or blocked.

More to Know

Normally, urine flows from the kidneys to the bladder through thin tubes called ureters. But some conditions and obstructions in the urinary tract can block the normal flow of urine or reverse the urine flow. This can cause a buildup of urine in the kidney, leading to hydronephrosis, or swelling of the kidneys. Hydronephrosis sometimes can be diagnosed before a baby is born on prenatal ultrasound.

Hydronephrosis may lead to a urinary tract infection or cause belly pain. The two main causes of hydronephrosis include blockage of urine flow (which can happen at different points in the urinary tract) and urine reflux (a reversal of urine flow).

Keep in Mind

Many times, hydronephrosis clears up without treatment. If treatment is needed, most causes of hydronephrosis can be resolved with medicines or minor surgery.

All A to Z dictionary entries are regularly reviewed by KidsHealth medical experts.