Prenatal Hydronephrosis (Renal Pelvic Dilation)

One of the most significant urological problems falling under the scope of pediatric urology is hydronephrosis, which refers to the enlargement of the kidney. Hydronephrosis signifies an expansion in the collecting system of the kidney. It constitutes 50% of all congenital urinary system anomalies. With the routine use of fetal ultrasound today, most hydronephrotic kidneys receive a diagnosis during maternal ultrasound examinations in the prenatal period.

How common is Prenatal Hydronephrosis?

The incidence of hydronephrosis in all pregnancies is around 1-3%. Considering an estimated average of 1,300,000 births annually in our country, this means there are between 13,000 to 39,000 babies born with hydronephrosis. Hydronephrosis is more common in males than females. While it is more frequently observed on the left side compared to the right, 20-40% of cases are bilateral or involve both sides.

Prenatally detected hydronephrosis is often temporary or physiological. Therefore, in many cases, hydronephrosis resolves spontaneously without requiring any surgical intervention.

Causes of Hydronephrosis:

Ureteropelvic junction obstruction

Ureterovesical junction obstruction

Vesicoureteral reflux

Duplex ureter

Ectopic ureter

Ureterocele

Posterior urethral valve

Hydronephrosis is not synonymous with obstruction. 

Obstruction refers to a blockage in the urinary flow that hinders kidney development or causes damage to the kidney. Ureteropelvic junction obstruction (UPJO) is the most common cause of hydronephrotic kidneys requiring surgical treatment (40%). The most common reasons for bilateral hydronephrosis are posterior urethral valve in males and obstructive ectopic ureterocele in females.

Prenatal USG is crucial for early diagnosis!

Advanced ultrasound examinations and evolving technologies enable the detection of hydronephrotic kidneys starting from the second trimester. A renal anteroposterior (AP) diameter of 4 mm or more is considered hydronephrosis in the second trimester, and 7 mm or more in the third trimester. Severe hydronephrosis is labeled when the AP diameter is above 15 mm in the third trimester. Of the identified hydronephroses, 56-88% are mild, while 1.5-13.5% are severe.

What to do after detecting prenatal hydronephrosis?

Babies identified with prenatal hydronephrosis should undergo a postnatal urinary system ultrasound to examine the kidneys. However, due to the physiological nature of newborns, where urine production is low in the first three days, to avoid incorrect results, the follow-up ultrasound should be performed three days after birth.

Babies with diagnosed hydronephrosis require pediatric urology follow-up.

For a baby diagnosed with prenatal hydronephrosis, if the hydronephrosis is unilateral, a detailed urinary system ultrasound should be repeated on the third postnatal day. If it is bilateral, the repeat ultrasound should be done on the first day, and consultation with pediatric urology is recommended.

Hydronephroses usually resolve within the first 2 years of life.

While transient hydronephroses typically disappear within the first two years of life, most hydronephroses requiring surgery manifest within the initial 2 years. Studies comparing surgical risks with prenatal and postnatal AP diameters of hydronephrotic kidneys have shown that 12% of the mild group, 45% of the moderate group, and 88% of the severe group carry surgical risks.