Hydronephrosis Symptoms & Causes

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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


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