Prenatal Hydronephrosis

Prenatal hydronephrosis at a glance

  • Prenatal hydronephrosis is the swelling of a kidney in a fetus or infant caused by a buildup of urine in the kidney due to poor flow or blockage.
  • The two most common reasons for this condition are a narrowing or blockage of the junction where urine should exit the kidney or urine flowing back into the kidney rather than out to the bladder.
  • Prenatal hydronephrosis is the most common abnormality detected in maternal ultrasounds, affecting about 1 in 100 pregnancies.
  • Prenatal hydronephrosis does not usually cause problems for the fetus, and after birth, many babies require no treatment to achieve the normal flow of urine.
  • When treatment is needed, it can involve observation, antibiotics to prevent kidney infection or surgery in severe cases.

What is prenatal hydronephrosis?

Hydronephrosis is swelling (dilation) of the section of the kidney that stores urine. Hydronephrosis means “water on the kidney.” Hydronephrosis can affect babies both before (prenatal or fetal) and after (postnatal) birth. It is detected in about 1.4 percent of the 3 million maternal ultrasounds done each year in the U.S. This makes it the most common problem found in maternal ultrasounds, comprising 50 percent of the abnormalities detected.

In prenatal hydronephrosis, the blockage or inadequate urine flow causes a malfunction of the kidney’s urine collection system. This system normally collects urine from kidney tissue and sends it via the ureter tube to the bladder.

The most common cause of prenatal hydronephrosis is an obstruction at the ureteropelvic junction (UPJ). This is where urine flows out of the kidney and into the ureter tube. The obstruction is most often due to a narrowing of that passageway.

Another frequent cause of prenatal hydronephrosis is a defect in the vesicoureteral valve that normally stops urine from flowing back into the kidney, a condition called vesicoureteral reflux (VUR).

Most cases do not cause problems for the fetus. For most babies born with prenatal hydronephrosis, their condition resolves over time and they require no treatment to pass urine normally. But more serious cases of prenatal hydronephrosis can result in urinary tract infections, scarring or permanent kidney damage. Early detection can prevent these complications.

Diagnosis of prenatal hydronephrosis

Prenatal hydronephrosis is most often discovered by an ultrasound, performed either as a normal maternal imaging evaluation of the fetus or in evaluating a pregnant woman for another medical condition. The ultrasound of the fetus may show fluid buildup in one or both kidneys or ureter tube; a ureterocele (an abnormal flap of tissue in the bladder where the ureter enters it); or an enlarged bladder.

Ultrasound alone does not provide a detailed enough image of prenatal hydronephrosis, and further tests are often required for a proper evaluation. These tests include:

  1. Voiding cystourethrogram (VCUG), an x-ray evaluation of the bladder while it is emptying. This reveals important data on the shape and size of the bladder, the bladder neck and the ureter tubes. The test can also diagnose vesicoureteral reflux, as well as detect abnormalities that may cause a blockage in the urethra, which carries urine out of the body through the penis or the vagina,
  2. Renal scan, a diagnostic test in which a harmless radioactive material is injected into the bloodstream and observed as it is absorbed by normal kidney tissue. The scan reveals the functioning and drainage of the kidneys, as well as the presence of kidney damage or scarring that may have occurred.

Treatment of prenatal hydronephrosis

After prenatal hydronephrosis is detected in the fetus, the mother should see a pediatric urologist between weeks 20 and 30 of the pregnancy to monitor its condition. Chances are good that the condition will disappear either before birth or in the baby’s first few months after birth.

The extent of the prenatal hydronephrosis determines which treatment plan the urologist will implement to treat the underlying cause. Prenatal hydronephrosis is graded on a scale of I to IV, with I being no hydronephrosis and IV being a severe case.

Many cases of grade I, II or III prenatal hydronephrosis resolve over time and do not require treatment. The amount of time it takes for the problem to resolve varies according to the individual patient and the extent of the condition. Prenatal hydronephrosis is often treated with antibiotics to prevent kidney infections. The specific antibiotics used have very few side effects.

In a small portion of cases, usually grade IV prenatal hydronephrosis, surgery may be needed to correct the problem. This resolves the underlying issue in the great majority of cases.