To understand this condition, it is helpful to understand how the urinary tract works. In simple terms, the kidneys filter the blood and remove waste products that are then taken out of the body in the urine. The urine collects in the pelvis, which empties into a tube (the ureter) and then drains into the bladder. From the bladder, the urine drains out of the body through the urethra. 

During pregnancy, the placenta does most of this work for the baby. The baby’s kidneys produce urine starting as early as the fifth week of gestation. While the baby is inside the womb, the urine produced by the baby's kidneys adds to the amount of amniotic fluid (fluid surrounding the baby in the uterus). The fluid is important to help the lungs develop. It also gives the baby a "cushion" and provides him or her space to move.

Twenty to 30 percent of birth defects found before babies are born involve the urinary tract. Fifty percent of these babies have a condition called hydronephrosis.

Hydronephrosis occurs when the pelvis becomes enlarged because urine is collecting in the area of the kidneys. Doctors can diagnose hydronephrosis when the enlargement exceeds 10 mms at 20 to 24 weeks of pregnancy. 

Hydronephrosis can be the result of:

• A blockage, which can occur in a variety of places along the urinary tract
• Reflux or backward flow of the urine 
• Immaturity, which allows more stretching of the pelvis than normal
• An extra ureter (the tube that carries urine from the kidneys to the bladder)
• Multicystic kidney (a birth defect in which the kidney does not function)

When the pelvis is stretched or enlarged, but not enlarged enough for doctors to diagnose hydronephrosis, it is considered pyelectasis. Pyelectasis also is known as renal pelvic dilatation. (Dilatation means stretching or enlargement). The amount of stretching of the renal pelvis with pyelectasis is typically defined as greater than 4 mm but less than 10 mm in a baby younger than 24 weeks of gestation. Enlargement of 4 to 10 mm also may be called mild hydronephrosis. 

In 90 percent of cases, this condition will improve by itself and never become hydronephrosis. However, in 10 percent of cases, the dilatation will increase and hydronephrosis will be diagnosed. 

If your baby has evidence of pyelectasis or mild hydronephrosis, your doctor will want to perform another ultrasound as your pregnancy progresses. This is to examine your baby’s urinary tract. 

This ultrasound will look at: 

• The amount of dilatation in the pelvis
• The appearance of each kidney
• How many kidneys are affected
• Your baby’s overall growth
• Your baby’s gender
• The amniotic fluid index (the amount of amniotic fluid present)
• Bladder size and thickness
• How well your baby is emptying his or her bladder 

Doctors usually diagnose hydronephrosis on a routine ultrasound. If your baby is diagnosed with hydronephrosis, you will need to have follow-up ultrasounds to track the condition. About 85 percent of infants who are diagnosed with mild hydronephrosis before birth have an abnormal urinary tract. The other 15 percent of these infants will get better on their own and have no problems after birth. 

Of the 85 percent of babies with a defect, only 15 to 25 percent require surgery to correct it. Amniotic fluid volume is the single most important factor that shows the well-being of the unborn baby. Another finding that causes concern is an enlarged bladder.

Your doctor likely will refer you to a maternal-fetal medicine specialist (a doctor who handles high-risk pregnancies).

Other specialists you may see during pregnancy include a pediatric urologist (a doctor who specializes in the urinary system) or nephrologist (a doctor who specializes in the kidneys) and a neonatologist (a doctor who specializes in treating newborns). They will make recommendations for follow-up care during your pregnancy as well as follow-up care for your baby once he or she is born.

Pyelectasis or mild hydronephrosis will likely have little or no effect on your baby. Most babies with this condition do very well. Very rarely, a baby will have severe bilateral hydronephrosis or an extremely distended or filled bladder and insufficient amniotic fluid. These babies will have a more guarded prognosis (see the chapter on bladder outlet obstruction for more information).

How hydronephrosis affects your baby will depend upon its cause. Two of the more common causes for mild hydronephrosis and their effects are:

• Ureteropelvic junction obstruction, also referred to as UPJ obstruction, is the most common cause of hydronephrosis. With UPJ obstruction, the flow of urine from the kidney to the ureter is blocked. This can affect one or both kidneys. Complete obstruction, very early in the pregnancy (8 to 10 weeks) will result in severe dysplastic changes. If the UPJ obstruction is on one side only and has little effect on kidney function, we recommend testing after your baby is born. Your doctor will perform an ultrasound of the kidneys and bladder when your baby is about 4 weeks of age to determine if the hydronephrosis is still present. If your doctor suspects UPJ, he or she may perform a renal scan to confirm the diagnosis. This scan measures the kidneys' ability to make and drain urine. 
  
  If the initial ultrasound shows severe hydronephrosis and/or other changes, such as thickening of the cortex (the part of the kidney that produces urine), your doctor may perform a voiding cystourethrography (VCUG) test. This is done to see if urine is backing up. 
  
  Some babies with prenatally diagnosed hydronephrosis may be prescribed antibiotics after birth to prevent a urinary tract infection. When the baby is diagnosed with hydronephrosis before birth, follow-up is done soon after the baby is born.
  
  In the past, without diagnosis before birth, these babies with no apparent problems would go for years without a diagnosis, and kidney function could be severely affected. Surgery may be required if kidney function is affected. Pyeleoplasty (removal of the blocked area) is the surgery needed for UPJ obstruction. However, many cases of UPJ obstruction will get better on their own in the first 18 months of life. 
• Vesicoureteral reflux (VUR) is the abnormal flow of urine from the bladder back toward the kidney(s). This can result from an abnormal flap valve. The back flow of urine allows bacteria from the bladder to enter the kidney. This makes these babies more prone to urinary tract infections of the bladder and/or kidney(s) (pyelonephritis). 
  
  Pyelonephritis can seriously damage the kidney(s). This can put babies at risk for hypertension and kidney failure.Vesicoureteral reflux is diagnosed through a VCUG test or radionuclear cystourethrogram (RNC) test. If reflux is found, your doctor may perform an isotope renal scan to evaluate kidney function and assess for damage.
  
  When the diagnosis of reflux is made early, treatment is aimed at preventing urinary tract infections or UTI. Treatment may include:
  • Antibiotics
  • Physical exams 
  • X-rays of the bladder and/or kidneys (to monitor the reflux)
  • Urine analysis (to check for infections) 

Often, reflux will disappear as the child grows and the ureter lengthens and develops. This form of treatment is most commonly used for reflux that causes only mild hydronephrosis and is less severe.

Surgery is another possible treatment. It aims to fix the flap valve problem so that urine is not able to flow backward. It also may fix a twisted ureter or dilated/distended ureter. Surgery is used when reflux causes more severe hydronephrosis that is more likely to result in kidney damage.

Doctors usually diagnose hydronephrosis during a routine ultrasound. If your doctor suspects that your baby has hydronephrosis, he or she will refer you for a follow-up ultrasound that can examine your baby’s anatomy in more detail.

The two most important factors for babies who have hydronephrosis are the volume of amniotic fluid and the appearance of the kidneys. These factors can change for either better or worse as the pregnancy progresses. For that reason, you may need to have multiple ultrasounds to watch for changes in symptoms. 

How hydronephrosis affects your pregnancy will depend upon the severity of the condition. Most women’s pregnancies progress normally. 

Your doctor will use ultrasound to observe your baby's kidneys to ensure they continue to function well. Your doctor also will monitor the amniotic fluid volume, which is the best clue about how well your baby’s kidneys are functioning.

Most babies with hydronephrosis will be cared for in the newborn nursery. Some may have an ultrasound of their kidneys and bladder before they go home. Most will have an ultrasound at approximately 4 weeks of age. These babies usually go home when their mother is discharged, and doctors will schedule the ultrasound for a later date. 

Even if the first ultrasound after birth is normal, your baby will have to have another one later to make sure that the hydronephrosis hasn't returned. That said, it is rare for mild enlargement to progress. The majority of babies diagnosed with mild hydronephrosis before birth will require no type of treatment except observation. 

If the hydronephrosis continues to be seen after birth or if an ultrasound shows there are changes in the kidney(s), your doctor will order tests to determine if your baby has reflux or an obstruction. An X-ray may be taken to look more closely at the renal anatomy, and other tests may be done to rule out reflux. If your baby has an obstruction or reflux and it is causing problems with kidney function, he or she may need surgery.

Rarely, surgery to fix an obstruction or reflux is required for mild hydronephrosis.

Surgery is almost never needed when the baby is first born. Instead, it is scheduled after the child has grown and tests have been done to measure the extent of the problem.

Yes. Your baby will more than likely go to the newborn nursery and be treated there if hydronephrosis is his or her only problem. The urologist/nephrologist may see him or her in the hospital if you deliver at Froedtert & The Medical College of Wisconsin Froedtert Hospital Campus. If you do not deliver at Froedtert or the urologist/nephrologist does not see your baby before you go home, please call to set up a follow-up appointment soon after you take your baby home. 

After birth and before your appointment with a pediatric urologist, an ultrasound of the kidneys will be done to look at the structures. This is also to compare the pictures taken before your baby was born. 

Long-term prognosis is excellent for most babies with hydronephrosis. Even if your baby has only one working kidney, he or she can live a full life with few limits on activity.

For more information, visit the Urology Care Foundation.