What is hydronephrosis?

What parents need to know about the ‘swelling’ of the kidneys

“Your baby has hydronephrosis.”

Many expecting parents hear this from their obstetrician after a routine screening maternal-fetal ultrasound. It is estimated that three to five million maternal-fetal ultrasounds (about 2 percent) detect hydronephrosis.

Hydronephrosis is often called “fetal,” “prenatal,” or “antenatal,” and many times just referred to as “swelling” of one or both kidneys.

Kidneys essentially have two parts – the parenchyma and the collecting system. The parenchyma filters the bloodstream and the collecting system brings this filtered urine to the bladder to be urinated out.

During pregnancy the mother’s placenta filters the baby’s bloodstream, but the kidneys still make urine. All developing babies urinate inside the pregnant uterus. This “amniotic fluid” is vital to the baby’s development. Low amniotic fluid can mean poorly functioning kidneys or a blocked collecting system. If the collecting system is blocked or there is back-up of urine from the bladder, the kidney(s) will begin to swell. This is known as hydronephrosis.

When hydronephrosis is detected, specialists routinely check other ultrasound findings to help determine the potential risk to the kidney(s). These are the severity or “grade” of hydronephrosis, the volume or “index” of amniotic fluid, the presence or absence of dilation of the ureter (the tube that carries the urine form the kidney to the bladder), the size or “distention” of the bladder, thickness of the bladder wall, emptying of the bladder, the integrity or thickness of the kidney parenchyma, or the presence of kidney cysts.

The most common cause of prenatal hydronephrosis is an obstruction or narrowing where the ureter meets the kidney.

When a dilated ureter is detected as well, the problem may be urine backing-up into the kidney from the bladder, or an obstruction where the ureter meets the bladder.

Parents should know that approximately 85 percent of prenatal hydronephrosis is mild to moderate and resolves after birth by an average of 14 to 18 months of age.

Most of these children are otherwise healthy. The other 15 percent require more detailed surveillance and possible medical and/or surgical treatment. The ability to accurately predict this higher risk group in-utero is not very accurate. After birth, a non-invasive repeat ultrasound is performed at one week, after a period of natural dehydration/weight loss, so as not to underestimate the severity of hydronephrosis.

In many cases, the pediatrician or specialist may start a low dose antibiotic to prevent infection. In some cases a voiding cystourethrogram (VCUG) is needed. This involves placing a catheter into the urethra to fill the bladder with X-ray dye to see if there is vesicoureteral reflux.

During the period of uncertain diagnosis, and before resolution or treatment, parents may become discouraged and confused. Parents should be patient, vigilant about medical recommendations, knowledgeable about the overall risks and ask questions. However, most importantly parents should not forget to enjoy the birth of their new baby.

Daniel B. Herz, MD, FAAP, FACS is the Program Director of Pediatric Urology and Associate Professor of Urology and Pediatrics at the Children's Hospital at Dartmouth-Hitchcock. For more information, go tochadkids.org.

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