Children's Hospital Boston explains that normally urine in the body flows from the kidneys down through the ureters and into the bladder. When something unusual happens in the urinary tract that prevents the urine from draining into or out of the bladder the kidney becomes overly full and dilates, a condition known as hydronephrosis. Prenatal hydronephrosis is the medical term for this occurring in a fetus. A prenatal ultrasound can help diagnosis this condition before birth.
Ureteropelvic Junction Blockage
While in the womb, an ultrasound can reveal hydronephrosis in a fetus but it may not be able to identify the exact cause. Once the baby is born, further examination can help reveal the underlying cause of the condition. Children's Hospital Boston explains that a partial or complete blockage of urine can lead to hydronephrosis. A partial blockage is one that allows urine to pass but at a slower rate and with increased pressure.
A blockage at the ureteropelvic junction is the most common type that leads to hydronephrosis. The ureteropelvic junction is the area where the ureter joins with the kidneys. The blockage at this site can occur due to an abnormal narrowing at the top of the ureter. During a routine prenatal ultrasound an infant with ureteropelvic junction blockage that has caused hydronephrosis will exhibit swelling in the pelvic region but not in the ureter.
Vesicoureteral Reflux
Vesicoureteral reflux is a condition that occurs when urine resting in the bladder flows back into the ureters or even back into the kidneys, according to Children's Hospital Boston. This is the opposite of the direction urine should flow. The result is the swelling of the kidneys as urine pools inside.
Ureterovesical Junction Blockage
The ureterovesical junction is the area where the ureter joins with the bladder. The Children's Hospital Boston points out that this is the second most common type of blockage that causes hydronephrosis.
Other Blockages
Other areas where a blockage may occur include the connecting of the ureter with the bladder or in the urethra, which empties into the bladder, according to Children's Hospital Boston. These blockages can be due to an abnormal flap of tissue in the urethra, which will lead to bladder bloating, kidney swelling and ureter swelling that can be seen on an ultrasound, according to UPMC in Pittsburgh. In some cases there will be a decrease in the level of amniotic fluid that surrounds the baby.
Vesico-ureteral Reflux
Vesico-ureteral reflux is a condition in some children that causes urine to flow backwards from the bladder into the kidneys, resulting in hydronephrosis, according to the Children's Hospital Boston. An ultrasound will reveal swelling in the kidneys that ranges from mild to severe but other signs to aid in diagnosing the cause will not be present.
