Causes of Vesicoureteral Reflux
1. Primary vesicoureteral reflux
Most children who have VUR have primary VUR, which means they are born with an abnormal ureter. With primary VUR, the valve between the ureter and the bladder does not close well, so urine comes back up the ureter toward the kidney. If only one ureter and one kidney are affected, doctors call the VUR unilateral reflux.

Primary VUR can get better or go away as a child gets older. As a child grows, the entrance of the ureter into the bladder matures and the valve works better.

2. Secondary vesicoureteral reflux
Children can have secondary VUR for many reasons, including a blockage or narrowing in the bladder neck or urethra. For example, a fold of tissue may block the urethra. The blockage stops some of the urine from leaving the body, so the urine goes back up the urinary tract.

A child also can have secondary VUR because the nerves to the bladder may not work well. The nerve problems can prevent the bladder from relaxing and contracting normally to release urine.

Children with secondary VUR often have bilateral reflux, meaning the VUR affects both ureters and both kidneys. Doctors can sometimes diagnose a urine blockage in a fetus in the womb.