Hydronephrosis occurs when urine builds up in the kidneys and cannot drain out to the bladder. The kidneys swell from the excess urine. The condition may affect one kidney or both. Hydronephrosis is not a disease itself. It is a sign of another disease or condition affecting the kidneys.

Kidney, Ureter, Bladder, and Kidney Stone
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Hydronephrosis is caused by one of two problems in the urinary system. A blockage may prevent urine from draining out of the kidneys. Or, a condition called reflux may cause urine to flow back into the kidneys from the bladder.

Conditions that may cause hydronephrosis include:

  • A blockage or defect in the urinary system that is present at birth
  • Kidney stones
  • A blood clot
  • Scarring of the ureters, which are the tubes that connect the kidneys to the bladder
  • A tumor in the pelvic area such as the bladder, cervix, colon, or prostate
  • Enlarged prostate
  • Enlarged uterus during pregnancy
  • Persistent urinary infection in the kidneys
  • Neurogenic bladder
  • Injury to structures in the urinary system, such as from surgery or trauma