Typically, urine passes from the kidney through a tube called a ureter that drains into the bladder, and then out of the body. But, sometimes urine backs up or remains inside the kidney or in the ureter. That's when hydronephrosis can develop. Some common causes of hydronephrosis include: Partial blockage in the urinary tract. Urinary tract blockages often form where the kidney meets the ureter. Less commonly, blockages may occur where the ureter meets the bladder. Vesicoureteral reflux. Vesicoureteral reflux happens when urine flows backward through the ureter from the bladder up into the kidney. Typically, urine flows only one way in the ureter. Urine flowing the wrong way makes it difficult for the kidney to empty properly and causes the kidney to swell.