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What is hydronephrosis?

Hydronephrosis is a condition, affecting about 1 in 100 babies, where urine overfills or backs up into the kidney, causing the kidney to swell. Infants with hydronephrosis may be diagnosed before (prenatal) or after (postnatal) birth.

In many of the children who are diagnosed prenatally, the condition disappears spontaneously by the time of birth or soon after. In children who have mild or sometimes moderate hydronephrosis, kidney function is commonly unaffected and the condition may resolve over a period of time after delivery.

Hydronephrosis affects the drainage of urine from the urinary system — the kidneys, ureters, bladder, and urethra. When the urinary system is impaired, this can cause the urine to back up and the kidney to swell. Typically, hydronephrosis is caused by either something blocking urine flow or by urine leaking backward through the urinary system (reflux).

Your doctor will describe your child’s hydronephrosis as mild, moderate, or severe — based on how much the kidney is stretched and how much the urinary flow is impaired — and will tell you whether your child’s hydronephrosis affects one kidney (unilateral) or both kidneys (bilateral). More than half of the cases resolve by the time the baby is born or soon after.

Hydronephrosis | Symptoms & Causes

What are the symptoms of hydronephrosis?

Most babies with hydronephrosis have no symptoms. Older children may also have no symptoms if they have mild or moderate hydronephrosis, and the condition may disappear on its own.

If your child has moderate to severe hydronephrosis, some symptoms may include:

A child with hydronephrosis may develop a UTI. Symptoms of a urinary tract infection can include the following:

  • strong urge to use the bathroom
  • painful urination
  • cloudy urine
  • back pain
  • fever
  • vomiting

If your infant has had multiple urinary tract infections (UTIs) with (or without) a fever, it could indicate some kind of obstruction or reflux in the urinary system. However, UTIs can be difficult to spot in infants: In many cases, multiple, unexplained fevers are the only sign.

Older children may have more recognizable symptoms of UTIs, including a strong urge to urinate, painful urination, or cloudy urine. If your child tends to get repeat UTIs, you may want to have him evaluated for possible urinary tract obstruction.

What are the causes of hydronephrosis?

Two types of problems cause hydronephrosis. One is obstruction, where urine is physically prevented from draining out of the kidney. The obstruction, or blockage, can occur at any point in the urinary system from the kidney down to the urethra. The second is reflux, in which urine flows back up into the kidney.

Blockage (obstruction)

  • Ureteropelvic junction (UPJ) obstruction: A blockage at the point where the kidney joins the ureter (the thin tube that carries urine to the bladder). A narrowing at the top of the ureter is usually the cause.
  • Ureterovesical junction (UVJ) obstruction: A blockage at the point where the ureter joins the bladder.
  • Posterior urethral valves (PUV): A congenital condition, found only in boys, in which there are abnormal flaps of tissue in the urethra, causing bladder obstruction. This type of obstruction is also associated with vesicoureteral reflux.
  • Ureterocele: A bulge in the ureter that can obstruct part of the kidney and sometimes the bladder.
  • Vesicoureteral reflux (VUR): A backwash of urine that happens when the muscles at the junction of the ureter and bladder aren’t working properly and allow urine to flow back up toward the kidney with bladder filling or emptying.

Other causes of hydronephrosis

  • Ectopic ureter: A rare condition where a ureter doesn’t connect to the bladder in the normal location.
  • Unknown: In more than half of the children who are prenatally diagnosed with hydronephrosis, the condition resolves itself and the cause is never known.

Hydronephrosis | Diagnosis & Treatments

How is hydronephrosis diagnosed?

The first step in treating your child is forming an accurate and complete diagnosis. Hydronephrosis isn’t a disease. Instead, it indicates an impairment in your child’s urinary flow causing a kidney to swell. In diagnosing hydronephrosis, your doctor will search for what is causing the impairment to determine the best treatment for your child.

You may hear your child’s hydronephrosis described as:

  • “prenatal” or “antenatal” — meaning before the birth
  • “postnatal” or “neonatal” — meaning after the birth

These terms simply name when the diagnosis happened. (They are not saying anything about the condition itself.)

Prenatal testing

Signs of hydronephrosis may first show up on a routine prenatal ultrasound (sonography). The test transmits high-frequency sound waves into the uterus. The echoes that bounce back are recorded and made into an image of your baby. It will show the size and shape of your child’s kidneys, as well as the amount of amniotic fluid. It also can help reveal obstructions in the urinary system.

However, doctors usually can’t make a precise diagnosis of hydronephrosis based on ultrasound findings.

If an ultrasound indicates your child may have hydronephrosis, your obstetrician will monitor your pregnancy more closely and may perform more frequent ultrasound testing to check for any changes over time. While your child is being monitored, your caregivers will take precise measurements of your child’s kidneys and the level of amniotic fluid.

Postnatal testing

For newborns and older children, doctors may use some or all of the following tests to help determine the cause and nature of the hydronephrosis:

  • Renal ultrasound (RUS): By focusing on the kidneys, this ultrasound gives a good picture of the hydronephrosis. This is the first postnatal test your doctor will perform and will help determine whether further studies are needed.
  • Voiding cystourethrogram (VCUG): This special kind of x-ray is used to check for reflux, a common cause of hydronephrosis. It also may show an obstruction in the urethra. Using a small tube called a catheter, doctors will fill your child’s bladder with a liquid dye containing iodine. As the bladder fills and your child urinates, the flow of the liquid will be visible on x-ray images.
  • Renal scan (MAG 3): This test allows doctors to see your child’s kidneys and learn more about how they are functioning. This kidney scanning test helps measure the difference in function between the two kidneys and also estimates the degree of blockage in the urinary system. After a tiny amount of radioactive material (radioisotope) is injected into your child’s bloodstream, a special camera (called a gamma camera) is used to take pictures of the kidneys as the radioactive material moves through them, showing how well they are filtering and draining.

After we complete all necessary tests, our experts meet to review and discuss what they have learned about your child's condition. Then we will meet with you and your family to discuss the results and outline the best treatment options.

What are the treatment options for hydronephrosis?

After considering the nature and cause of the hydronephrosis, doctors will decide between recommending observation or surgery.

Fetal intervention

In very rare instances, prenatal hydronephrosis is so severe that it puts the fetus at risk. This usually means the obstruction is in the child's urethra, blocking drainage of the bladder and both kidneys. In turn, this results in a dangerously low amount of amniotic fluid (a condition called oligohydramnios).


If postnatal testing shows your child has mild to moderate hydronephrosis, your doctor may recommend allowing time for the condition to correct itself on its own. Your child may receive a low dose of antibiotics to prevent infection. Repeat ultrasounds will let us check for improvement.

Observation has become the accepted method of treatment in children with mild hydronephrosis. Even in children with moderate hydronephrosis, if kidney function is not lost and kidneys are growing well, observation can allow the condition to correct itself.


Only in severe cases would surgery be needed. The goal of the operation is to reduce the swelling and pressure in the kidney by restoring the free flow of urine.

The most common surgical procedure is pyeloplasty. This repairs the most common type of blockage that causes hydronephrosis: ureteropelvic junction obstruction (UPJ). In pyeloplasty, the surgeon will remove the narrowed or obstructed part of the ureter. Then, the healthy portion is reconnected to the kidney's drainage system. After open surgery (small incision over the kidney), children usually stay in the hospital for about two to three days. They heal in two to three weeks. The success rate is about 95 percent.

Other surgical treatments may be recommended for your child, depending on what's causing the hydronephrosis and how severe it is. To learn more about these, see: ureteropelvic junction obstruction, vesicoureteral reflux, posterior urethral valves, and ureteroceles.

Robot-assisted and minimally invasive surgery

This innovative tool is used for about half of the pyeloplasties performed by our urological team.

A robot-assisted pyeloplasty is a minimally invasive laparoscopic procedure. With the use of a tiny camera, surgeons operate using very thin instruments inserted into three or four small incisions. Robot-assisted pyeloplasty removes an obstructed section of the ureter and reattaches the healthy portion to the kidney's drainage system.

Robotic surgery can offer a number of benefits as compared to traditional (open) surgery, including:

  • less discomfort after the operation
  • smaller scars on the belly
  • a shorter hospital stay — usually 24 to 48 hours
  • quicker recovery
  • earlier return to full activities

Even if pyeloplasty is recommended for your child, a robot-assisted procedure may or may not be suitable. Your doctor will recommend the best options for your child.

How we care for hydronephrosis

At Boston Children’s Hospital, we are here to help. Our physicians and nurses are trained in pediatric urology and have extensive experience with hydronephrosis. We are honored to be recognized by U.S. News & World Report as one of the best children’s hospitals in the nation, and have the largest pediatric urology service in the world.

Hydronephrosis | Programs & Services