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  • Hydronephrosis is a condition where urine overfills, or backs up, into the kidney, which causes the kidney to swell. Infants with hydronephrosis may be diagnosed before or after birth.

    If your baby is diagnosed with hydronephrosis, here are a few helpful things to know:

    • 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.
    • We are here to help. At Boston Children’s, our physicians and nurses are trained in pediatric urology and have extensive experience with hydronephrosis. We are ranked #1 in the nation and have the largest pediatric urology service in the world.

    Key facts

    • 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.
    • Hydronephrosis affects about 1 in 100 babies.
    • Hydronephrosis is typically caused by either something blocking urine flow or by urine leaking backward through the urinary system (reflux). Identifying the cause of your child’s hydronephrosis will help determine how we recommend treating it.
    • Your doctor will describe your child’s hydronephrosis as mild, moderate or severe. This description is based on how much the kidney is stretched and how much the urinary flow is impaired. Your doctor will tell you whether your child’s hydronephrosis affects one kidney (unilateral) or both kidneys (bilateral).
    • Hydronephrosis can be detected via ultrasound. More than half of the cases resolve by the time the baby is born or soon after.
    • In children who have mild or, sometimes, moderate hydronephrosis, kidney function is frequently unharmed.
    • The likelihood of surgery depends on the cause and severity of your child’s hydronephrosis. Surgery for mild cases is unlikely. For moderate cases, surgery occurs 25 percent of the time. Children with severe hydronephrosis will need surgery 75 percent of the time.
    • If surgery is required, our success rate is 95 percent successful and unsurpassed.
    • Hydronephrosis is not always congenital. It can develop as a result of injury or other illness, such as kidney stones.
  • Boston Children’s expert urology team is dedicated to educating families about hydronephrosis and providing the best care for your child.

    What is hydronephrosis?

    Hydronephrosis is a condition where a backup of urine causes the kidney to swell. How far the kidney is stretched, and how much the urinary flow is affected, will tell us about the nature of your child’s condition.

    • Mild: Kidney function will be only slightly affected and, typically, the condition resolves itself.
    • Moderate: Kidney function is affected in some children.  The condition will be closely monitored.
    • Severe: Kidney function is likely affected. 

    In cases where hydronephrosis is diagnosed before the baby is born, the majority of children have no symptoms.


    How common is hydronephrosis?

    Hydronephrosis is a relatively common congenital condition (meaning a child is born with it), with some estimates showing it affects about 1 in 100 babies.

    Prenatal hydronephrosis is often spotted on routine prenatal ultrasounds or at our Advanced Fetal Care Center and is by far the most common urinary tract abnormality we deal with.

    Postnatal hydronephrosis is often diagnosed after a child experiences symptoms, such as pain or blood in the urine (hematuria). Symptoms may also result from a urinary tract infection. If kidney swelling is present, it can be detected by ultrasound.

    Who is at risk for hydronephrosis?

    There are no known risk factors, except that boys are four-to-five times more likely to be born with hydronephrosis than girls are. Hydronephrosis does not run in families.

    Neither hydronephrosis nor its underlying causes have been linked to anything parents did during pregnancy. In short, there is nothing you could have done to cause or prevent your child’s condition.

    When should I seek medical advice?

    Hydronephrosis is often discovered during routine prenatal ultrasound testing.

    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 complications are associated with hydronephrosis?

    The most common complication of hydronephrosis is UTI—specifically, kidney infection (pyelonephritis) caused by bacteria spreading from the bladder. In children with a severe form of the condition and/or when it affects both kidneys, complications can include kidney damage, kidney enlargement and sometimes kidney failure.

    What are the causes?

    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. See below.
    • 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

    • 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.

    Signs and symptoms

    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:

    • pain in the abdomen
    • pain in the side (flank pain)
    • blood in the urine (hematuria)

    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

    Prenatal hydronephrosis FAQ

    Q: How soon can hydronephrosis be seen on a prenatal ultrasound?
    A: A baby’s kidneys begin to produce urine about 10 to 12 weeks after conception, but it’s usually not until the fourth month or later that doctors can see signs of hydronephrosis.

    Q: Do I need to ask for a special kind of prenatal ultrasound to check for hydronephrosis?
    A: No. Doctors can look for signs of hydronephrosis and other conditions in routine ultrasounds during your pregnancy.

    Q: What can a prenatal ultrasound tell us about my baby’s condition?
    A: Ultrasound is extremely useful in detecting signs of hydronephrosis. However, fetal tissue is very elastic—which means a swelling in the kidney may look worse on ultrasound than the condition really is. That is why your doctor will keep a close eye on the size of the fetus and the kidneys, as well as the level of amniotic fluid, throughout your pregnancy.

    Q: Can prenatal hydronephrosis pose a serious risk to my baby?
    A: In most cases where a diagnosis of hydronephrosis is made in an unborn baby, no treatment is needed in pregnancy. The mother is monitored by a specialist in Maternal Fetal Medicine or Obstetrics. In very rare cases, the condition will impair the flow of fetal urine to the point that there will be too little amniotic fluid (oligohydramnios). Too little amniotic fluid can impact fetal growth and development.

    Q: If my baby has hydronephrosis, will I still have a normal delivery?
    A: In the vast majority of cases, a finding of hydronephrosis won’t have any effect on either your pregnancy or your delivery.

    Postnatal hydronephrosis FAQ

    Q: After I deliver my child, what happens next?
    A: Doctors may prescribe a small daily dose of antibiotics (amoxicillin), typically taken until your child’s first evaluation, within two to three weeks. Children with mild hydronephrosis will likely not require any medications.

    Q: Does this mean my other children will have hydronephrosis?
    A: No. Hydronephrosis doesn’t run in families and has never been linked to anything the parents did during pregnancy.

    Q: Will my child need dialysis because of hydronephrosis?
    A: Probably not. Dialysis is rarely needed in children diagnosed with hydronephrosis. Your child will not need dialysis if at least one kidney is functioning normally.

    Q: I’ve heard a lot about robotic-assisted surgery. Can I request that for my child?
    A: While robotic-assisted surgery can offer a number of benefits compared to traditional surgery, it isn’t recommended for every hydronephrosis patient. If your child needs an operation, your doctor will work with you to decide on the best surgical approach based on your child’s age and the underlying cause of her condition.

    Useful medical terms

    antenatal: before birth

    bilateral hydronephrosis: swelling affecting both kidneys

    hydroureter: a swelling of the ureter caused by excess urine

    kidney scan (MAG 3): a nuclear scanning test that helps measure the difference in function between the two kidneys and also estimate the degree of blockage in the urinary system

    neonatal: immediately after birth

    prenatal: before birth

    posterior urethral valves (PUV): a congenital condition in which there are excess flaps of tissue in the urethra, obstructing the bladder

    postnatal: after birth

    pyeloplasty: an operation that removes an abnormally narrow portion of the ureter and reconnects the remaining portion to the kidney’s drainage system. It is one of the most common operations for hydronephrosis.

    renal: related to the kidneys

    renal ultrasound (RUS): a safe and painless test that shows the size, shape and position of the kidneys

    unilateral hydronephrosis: swelling affecting one kidney (not both)

    ureter: a long, narrow tube that carries urine from the kidney to the bladder

    urethra: the tube through which urine travels from the bladder out of the body

    urinary system: made up of the kidneys, ureters, bladder and urethra; it filters waste from the body and removes it in the form of urine.

    unilateral: affecting one side

    bilateral:  affecting both sides

    ureterocele: a bulge in the ureter at the bladder level that can obstruct part of the kidney and sometimes the bladder

    ureteropelvic junction (UPJ) obstruction: a blockage in the urinary system at the point where the kidney joins the ureter

    ureterovesical junction (UVJ) obstruction: a blockage in the urinary system at the point where the ureter joins the bladder

    vesicoureteral reflux (VUR): an abnormal flow of urine from the bladder back into the ureter and sometimes the kidneys

    voiding cystourethrogram (VCUG): a type of x-ray that shows the flow of urine through a patient’s bladder and urethra during urination

  • 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.

  • Receiving news about your baby having kidney swelling may be worrisome. At Boston Children's Hospital, we view the diagnosis as a starting point for care and treatment. Once we have identified the cause of your child's condition, we can begin the treatment process with the goal of returning her to good health.

    If your baby is diagnosed with hydronephrosis, here are a few helpful things to keep in mind:

    • In many of the children who are diagnosed before birth, the condition disappears spontaneously before they are born or soon after.
    • In children who have mild or, sometimes, moderate hydronephrosis, kidney function isoften unharmed and the condition may resolve itself over a period of time after delivery.
    • If surgery is called for, we have a very high success rate
    • Only a handful of cases require treatment, or fetal intervention, while a baby is in the womb. For those babies who require treatment, it is nearly always provided after birth.

    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 the Boston Children's treatment sections for the following: ureteropelvic junction obstruction, vesicoureteral reflux (VUR), posterior urethral valves (PUV) and ureteroceles.

    Robot-assisted and minimally invasive surgery

    We are a nationally recognized pioneer in robotic-assisted surgery. Boston Children's was the first pediatric hospital to use a surgical robot, beginning in 2001.  

    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

    Note: 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.  

    Coping and support

    We understand that you may have a lot of questions when your child is diagnosed with hydronephrosis. Will it affect my child long term? What do we do next? In addition to our   website, we offer a number of other resources to help you and your family through this difficult time.

    Patient education: Our nurses are on hand to walk you through your child's treatment and help answer any questions you may have.

    Parent to parent: Do you want to talk with someone whose child has been treated for hydronephrosis? We can often put you in touch with other families who have been through the same experience you and your child are facing and can share their story.

    Social work: One important member of our pediatric urology team is a dedicated social work professional who has helped many families in your situation. We can offer counseling and assistance with issues such as coping with your child's diagnosis; dealing with financial difficulties; and finding temporary housing near the hospital if your family is traveling to Boston from another area.

    On our For Families and Patients website, you can read all you need to know about:

    • Getting Here – directions to Boston Children's
    • Accommodations
    • The hospital experience
    • Other resources that are available for your family

    In addition, two leading national groups provide additional information on hydronephrosis and may help connect you with parents across the country:

    • The National Kidney Foundation: This nonprofit, volunteer organization is dedicated to improving the health and well-being of anyone affected by kidney disease, preventing kidney and urinary tract diseases and promoting organ transplantation.
    • The American Association of Kidney Patients: This nonprofit organization is founded “by kidney patients for kidney patients” and is dedicated to educating and improving the well-being of people with kidney ailments.
  • In addition to leading the way in the use of robotic surgery, Boston Children’s Hospital is always working toward more effective treatments for children with urologic disorders. Our urology department has been conducting several projects that hold great promise for improving the lives of children with hydronephrosis.     

    Finding the genetic markers of vesicoureteral reflux (VUR): One of the more common causes of hydronephrosis, VUR has long been difficult for physicians to manage. In addition, it has often been frustrating for parents because there is currently no way to detect which children are at risk.

    To diagnose VUR, physicians must pass a catheter up the urinary tract into the bladder. Our researchers aim to replace that procedure with a simple genetic test. They also hope to offer a way to predict which children have VUR that will not resolve on its own.

    Using proteins to target kidney obstruction: A team led by Richard Lee, MD, is studying proteins in the urine to identify illnesses at an earlier age. Specifically, the team hopes to be able to predict risk for VUR. Another goal is to detect changes in the bladder tissue that may tell whether a child’s hydronephrosis is getting worse.

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