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(Hi-dro-nef-roe-sis) is a condition where urine overfills, or backs up, into the kidney, which causes the kidney to stretch (dilate), much like a balloon when it is filled with water. Infants with hydronephrosis may be diagnosed prenatally (before birth) or after birth during evaluation for other issues or after a urinary tract infection.

Hydronephrosis is very common. It affects about 1 in 100 babies. It may be caused by something blocking the urine flow somewhere along the urinary drainage tubes or due to urine back-flowing from the urinary bladder into the ureter (the tube that drains the kidney) and kidney.

Hydronephrosis can vary in severity. Typically, your doctor will describe your child’s hydronephrosis as mild, moderate or severe. Sometimes hydronephrosis is given a grade of 1, 2, 3 or 4, with 1 representing very minimal dilation and 4 representing severe dilation. Hydronephrosis may be present in one kidney (unilateral) or both kidneys (bilateral).

Signs and Symptoms

Infants and young babies with hydronephrosis have no symptoms. Older children may or may not have symptoms, depending on the cause of the hydronephrosis.

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If your child has symptoms, these may include pain in the abdomen or side (flank pain). They may or may not have blood in the urine (hematuria). Sometime they may have nausea or vomiting in addition to the pain.

Children with hydronephrosis may develop a UTI. The risk of urinary infection increases the severity of hydronephrosis. Symptoms of a urinary tract infection can include strong urge to use the bathroom, pain with urination, cloudy or smelly urine, back pain, and fever. If you child has hydronephrosis and experiences these symptoms, especially with a fever, please see your pediatrician.


Hydronephrosis is usually detected with an ultrasound but there is a different approach between pre- and post-natal diagnosis.

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Prenatal hydronephrosis: Hydronephrosis is often found on routine prenatal ultrasounds. It is the most common urinary tract abnormality found on prenatal ultrasound imaging. If the hydronephrosis is mild and your fetus is doing well, your doctor may not send you for a pediatric urology consultation. However, if the hydronephrosis is moderate-severe/grade 3-4, if your doctor is concerned, or if you would like to discuss your child’s condition, we are happy to see you in our office setting.

Postnatal hydronephrosis: Hydronephrosis is often diagnosed after a child has symptoms, such as abdominal or flank pain or blood in the urine (hematuria). If a child develops a urinary tract infection, an ultrasound is often performed. If hydronephrosis is present, it can be detected by ultrasound.


Learn more about the variety of causes that can lead to hydronephrosis.

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Hydronephrosis may be due to obstruction of the kidney drainage system, back-flow of urine from the bladder to the ureter and kidney or may simply be the way the kidney was formed during development. While radiology studies are needed to determine the cause, the severity of the hydronephrosis may be related to the reason for the dilation.

Obstruction: The kidney makes urine as it cleans the blood of toxins and manages the body’s water balance. Once urine is made, it flows into the kidney pelvis, down the ureter and into the bladder where it is stored. Once the bladder is full, it empties via the urethra. A blockage at any point along this pathway can lead to hydronephrosis.

Typically higher grades (moderate-severe, grades 3 and 4) of hydronephrosis are associated with obstruction. Sometimes if the obstruction is lower in the system, the ureter may be stretched (dilated). This is called hydroureter. Your child will most likely require a VCUG and a Mag 3 Lasix scan for evaluation. These studies may need to be repeated with or without follow up ultrasound imaging, depending on the results of the studies.

Vesicoureteral Reflux (VUR): The urinary system is designed for urine to flow “down-hill” from the kidney, down the ureter, into the bladder for storage. Once the bladder is full, it empties via the urethra. When the connection between the ureter and bladder is weak or abnormal, urine will flow “up-hill” from the bladder to the kidney. This stretches the kidney and results in hydronephrosis.

Unknown: In more than half of the children with hydronephrosis, the dilation resolves without any treatment. In these cases, the cause is never known.

Risk Factors

Learn about risk factors for hydronephrosis.

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To date, there are no known risk factors for hydronephrosis. However, boys are four-to-five times more likely to be born with hydronephrosis than girls. Hydronephrosis does not run in families, although some causes of hydronephrosis, such as VUR, may run in families. Hydronephrosis is not linked to anything parents did or did not do during pregnancy so there is nothing you could have done to cause or prevent hydronephrosis of your child’s kidney(s).

Hydronephrosis may be due to factors such as kidney stones, blood clots, tissue outgrowths (polyps) or other abnormalities. Typically treatment of these issues results in a normal appearing kidney and no hydronephrosis.


The most common complication of hydronephrosis is the development of a urinary tract infection (UTI).

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When the UTI is associated with a high fever, a kidney infection is (pyelonephritis) is suspected. This is caused by bacteria spreading from the bladder to the kidney and invading the kidney tissue. Children with severe hydronephrosis (moderate-severe, grade 3 and 4) have a higher risk of UTI/pyelonephritis. If pyelonephritis is severe or not treated in a timely manner, or if it affects both kidneys, complications such as permanent kidney damage (kidney scarring) can lead to problems such as high blood pressure and sometimes kidney failure.

Evaluation and Treatment

Discover how the physicians at UNC approach the evaluation and treatment of hydronephrosis.

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Evaluation: Identifying the cause of your child’s hydronephrosis will help determine how to follow or treat it. Depending on the severity of the hydronephrosis you doctor may recommend 1 or more of the following studies:

Voiding Cystourethrogram (VCUG): a type of radiology study to diagnose the backflow of urine from the bladder to the ureter and kidney

Mag 3 Lasix Scan: a type of radiology study to evaluate the kidney function and drainage

DMSA scan: a type of radiology study to evaluate the kidney function.

Treatment: Treatment options depend on the severity of the hydronephrosis and the result of the studies. However, in general, the severity of hydronephrosis often gives some clue about treatment.

Observation: If your child’s hydronephrosis is mild (grade 1 or 2) or if your child has moderate hydronephrosis and the kidney functions well and is growing normally, your child may undergo observation. During observation, your child is followed by an ultrasound to monitor for worsening hydronephrosis or to see if it improves or goes away. Your child may receive a low dose of antibiotics to prevent infection during observation.

Surgery: Surgery is recommended only in severe cases when kidney function is at risk either due to obstruction or recurrent UTIs. The goal of the operation is to open the obstruction. This will allow the urine to flow freely, relieving the pressure build-up in the kidney.

The type of surgery depends on the where the obstruction is in the urinary system. Your pediatric urologist will discuss where the obstruction is located and what operation is best to treat the obstruction. Typically incisions are 5 cm or less. Through this small incision, your surgeon will remove the obstructed area and reconnect the kidney’s drainage system. Children typically stay in the hospital for about two to three days. They heal in two to three weeks. The success rate is about 95 percent.

Robot-assisted and minimally invasive surgery: Robot-assisted surgery is a minimally invasive laparoscopic surgical procedure. Using a small camera and instruments inserted through 3-4 small incisions, your surgeon performs the surgery. Robot-assisted and minimally invasive surgery is not used for every type of surgery so it may or may not be the best option for your child. Your surgeon will recommend the best options for your child and discuss these options with you.

Should I be concerned?

If your baby or child is diagnosed with hydronephrosis, it can be worrisome and cause much anxiety. At UNC Pediatric Urology, our first goal is to diagnose why your child has hydronephrosis so that the best recommendations and treatment options can be determined.

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If your baby is diagnosed with hydronephrosis, there’s’ a few things to remember. First, many children who are diagnosed with hydronephrosis prenatally have no evidence of hydronephrosis before they are born or at follow up after birth. In most children who have mild and many children who have moderate hydronephrosis, kidney function is often normal, the kidney grows during follow-up and the condition resolves with time without any intervention.

Only a handful of children require surgery for hydronephrosis and most often, these children have or develop severe hydronephrosis with poor drainage of the kidney and sometimes, compromised kidney function.

If your child requires surgery, the overall success rate is around 95%, incisions are typically very small and children handle surgery well with mild-moderate discomfort.