In some fetuses and newborns with hydronephrosis, health care professionals can’t find a cause, and the hydronephrosis goes away on its own. This is called transient hydronephrosis.
In other cases, hydronephrosis is caused by a blockage in the urinary tract or reflux of urine from the bladder to the kidney.
Transient hydronephrosis
Experts think that narrowing of part of the urinary tract during early development may cause transient hydronephrosis. The condition resolves as the urinary tract matures.
About half of fetuses diagnosed with hydronephrosis have transient hydronephrosis, and the condition goes away before the infant is born. In children who are born with hydronephrosis, the condition is sometimes transient and can go away on its own, usually by the time the child is age 3.
Birth defects
Birth defects in the urinary tract may cause hydronephrosis. Even when birth defects are the cause, hydronephrosis may be mild and may improve as the child gets older. However, birth defects may also cause hydronephrosis that is severe or gets worse over time.
Ureter defects. Birth defects in the ureter that cause hydronephrosis include
- ureteropelvic junction (UPJ) obstruction, in which a blockage is present where the ureter joins the renal pelvis
- vesicoureteral reflux (VUR), in which urine flows backward, or refluxes, from the bladder to one or both ureters and sometimes to the kidneys
- other defects of the ureter or where the ureter joins the bladder, which may prevent the normal flow of urine from the kidneys to the bladder
Among all birth defects that cause hydronephrosis in fetuses and newborns, UPJ obstruction and VUR are most common.
Lower urinary tract obstruction. In lower urinary tract obstruction, also called bladder outlet obstruction, a blockage is present in the urethra or where the bladder joins the urethra. Birth defects that cause lower urinary tract obstruction include posterior urethral valves, which are abnormal folds of tissue that block the urethra. Posterior urethral valves only occur in males. Because lower urinary tract obstruction blocks urine flow from both kidneys, it is more urgent for doctors to examine lower urinary tract obstruction than blockage or reflux in a ureter.
Defects in the urinary tract and other parts of the body. Some infants with hydronephrosis have birth defects in the urinary tract and in other parts of their body. For example, prune belly syndrome is a group of birth defects that includes poorly developed abdominal muscles, undescended testicles, and urinary tract defects.
Birth defects that affect the spinal cord, such as spina bifida, can affect the nerves that control the urinary tract and cause urinary retention. Urine retained in the bladder may flow backward into the ureters and kidneys, causing hydronephrosis.