From the kidney, urine normally passes through the ureter and drains into the bladder before being flushed out of the body. However, when a blockage occurs at any part of the urinary tract or when there is urine reflux, the kidneys may swell. Hydronephrosis can be caused by: Blockages . These can occur: Where the kidney meets the ureter Where the ureter meets the bladder Within the urethra When the ureter is incorrectly attached to the bladder Backflow of urine into the kidneys . Also known as vesicoureteral reflux, this happens when urine incorrectly flows from the bladder back up into the kidney. As a result, the kidney does not empty itself, causing it to swell. Genetic reasons . This can occur with other conditions in the infant. Unknown reasons . This is known as idiopathic hydronephrosis.