Normally, urine flows from the kidneys down through the ureters and into the bladder. But, if there is some kind of a blockage that prevents urine from draining properly into the bladder or out of the bladder, the kidney overfills with urine and hydronephrosis occurs. The blockage may be partial, letting urine pass, but at a slower rate and with increased pressure.

Another cause of hydronephrosis is vesicoureteral reflux, when urine that dwells in the bladder flows back into the ureters and often back into the kidneys.

Most children with hydronephrosis are born with the condition although it can develop during childhood. It is the most common urinary tract anomaly (abnormality) and ranges in severity. In mild hydronephrosis, the pelvic (the part of the kidney that collects the urine) dilation is barely noticeable, whereas in severe hydronephrosis the swelling occupies much of the abdomen.

• It is four to five times more common in males than females.

• It can occur in one or both kidneys.

• Most mild cases and even some moderate cases may resolve on their own.

• More severe cases may require surgery.

It is unknown why some babies are born with the kinds of anomalies that lead to hydronephrosis. Neither hydronephrosis nor its causes have ever been linked to anything the parents did during pregnancy.

Not all children with hydronephrosis are born with it. Hydronephrosis can, for example, rarely develop as a result of stones or after an injury to the urinary tract or previous surgery.

In most cases, if hydronephrosis is detected on a prenatal ultrasound, you will be referred to a pediatric urologist for further evaluation and a more detailed ultrasound. Usually, a precise diagnosis cannot be made right away. Your pediatric urologist may want you to be monitored periodically to note any changes that occur over time, providing further evidence of a particular diagnosis or risk of affecting kidney function. While you are being monitored, doctors will be taking precise measurements of the fetus, fetus' kidney and the level of amniotic fluid.

At some point, the hydronephrosis will be classified as either mild, moderate, severe unilateral (occurring in one kidney) or bilateral (occurring in both kidneys.) Based on these classifications, the urologist will look for clues of any urinary tract anomalies that could cause the hydronephrosis.

Here are some examples of more common conditions responsible for hydronephrosis, but virtually any type of blockage can be responsible:

If doctors find moderate to severe hydronephrosis, severe unilateral hydronephrosis or severe bilateral hydronephrosis during the prenatal ultrasound, your child will receive antibiotics during the first days after birth. At one to three months of age, depending on the severity of the hydronephrosis, your child will undergo imaging studies to confirm the cause and determine if surgery is needed. Surgery is usually needed to correct the problem only in severe unilateral or severe bilateral hydronephrosis, although it may be considered in some moderate cases.

Surgery relieves the obstruction or reflux causing hydronephrosis. For descriptions of surgical treatments used to correct specific conditions causing hydronephrosis, see treatment sections for the following entries: ureteropelvic junction obstruction, ureterovesical junction obstruction, vesicoureteral reflux or posterior urethral valves.

Fetal intervention involves use of a vesicoamniotic shunt which surgeons, using ultrasound guidance, place in the fetus's bladder to drain it, bypassing the obstruction in the urethra. Urine is then allowed to drain into the amniotic space where the fetus is. The amniotic sac should normally be filled with urine from the fetus, but this does not occur with oligohydramnios from bladder obstruction. The most reliable fetal intervention technique is performed similarly to amniocentesis. The shunt passes through a large needle inserted through the mother's abdomen and wall of the uterus, directly into the fetus's enlarged bladder. As with all fetal surgeries, risks can include bleeding, infection or preterm labor, and should be discussed with your doctor.

Not all children with a severe degree of blockage do well, even with successful fetal intervention. This is because it can be difficult determine the extent of the damage to the fetus's kidneys and lungs (the lungs need the kidneys to develop normally) before the procedure. There will almost always be the need for some kind of surgical treatment after birth to allow bladder drainage and protect kidney function.