Congenital hydronephrosis involves dilatation of the renal pelvis and calyces. It is one of the most common diagnoses of renal anomalies made by prenatal ultrasound, is usually unilateral (75%) and shows a 5:1 preponderance in males. The most frequent cause is a urinary tract obstruction, such as a ureteropelvic obstruction (UPJ), uretero vesical obstruction (UVJ) or bladder outlet obstruction. Other anomalies which have given rise to hydronephrosis include vesicoureteral reflux (VUR), renal duplication with ureterocele formation, and retrocaval ureter. The embryology of this anomaly is not well understood. Hydronephrosis appears to occur relatively late in kidney formation. In experimental animal systems, ligature of the ureters after 80 days of gestation resulted in hydronephrosis.


Hydronephrosis can be unilateral or bilateral, mild, moderate, or severe. It may be transient and regress or progress as the pregnancy continues. The severity remains consistent in approximately 70%. Significant hydronephrosis should fit at least one of the following ultrasound findings: anteroposterior renal pelvic diameter of 10 mm. or more; a ratio of anteroposterior diameter of the renal pelves to the anteroposterior diameter of the kidney measured at the same level greater than 50%; the presence of calyceal dilatation; pelviectasis together with cystic change in the kidneys. Longstanding severe hydronephrosis results in permanent kidney damage; duration of the hydronephrosis is related to the degree of renal damage after birth. If bilateral hydronephrosis is present, oligohydramnios will often result; in unilateral cases amniotic fluid volume is expected to be normal. In some severe cases, rupture of the collecting system will result in paranephric urinoma or urinary ascites. Although urinomas may resolve spontaneously, the prognosis for kidney function is still poor. Unilateral cases of hydronephrosis, even severe ones, require only ultrasound follow-up if the contralateral kidney appears normal. In bilateral cases, follow-up scans should be initiated every 2-6 weeks to evaluate amniotic fluid volume as a measurement of renal function. Impairment of kidney function may require preterm delivery and surgical management in the newborn period.

Differential Diagnosis

Multicystic dysplastic kidneys (MCDK) exhibit a hydronephrotic” form which may also result from early urinary obstruction. There is however, more destruction of renal tissue with diffuse areas of cystic degeneration (noncommunicating cysts) as well as some renal parenchyma present. Renal pelvic dilatation does not involve the calyces. Transient mild hydronephrosis may be seen when the fetus has a full bladder; upon emptying, some kidneys will be considered normal.

Sonographic Features

Persistent excess fluid collection within renal pelvis

Caliectasis may be apparent in more advanced cases

Hydroureter may be apparent

If unilateral, amniotic fluid usually normal

If bilateral, there may be oligohydramnios

Associated Syndromes

  • Bladder exstrophy
  • Camptomelic dysplasia
  • Congenital hydronephrosis
  • Johanson-Blizzard
  • Ochoa
  • Post urethral valve syndrome (PUV)
  • Schinzel-Gierdion
  • Spondylocostal synostosis
  • Thanatophoric dysplasia
  • Triploidy
  • Trisomy 13
  • Trisomy 18
  • Trisomy 21


Romero R, Pilu G, Jeanty P, Ghidini A, Hobbins J In: Prenatal Diagnosis of Congenital Anomalies Appleton & Lange: Norwalk, p257,282
Nyberg D, Mahony B, Pretorius D In: Diagnostic Ultrasound of Fetal Anomalies, Text and Atlas Mosby Year Book: St. Louis, p731
Harrison MR, Golbus MS, Filley DA The Unborn Patient. Prenatal Diagnosis and Treatment Green & Stratton: New York, p277
Fleischer AC, Romero R, Manning FA, Jeanty P, James, AE Jr In: The Principles & Practice of Ultrasonography in Obstetrics & Gynecology, 4th Ed Appleton & Lange: Norwalk, p260-263
Crawfurd M díA In: The Genetics of Renal Tract Disorders Oxford U Press: Oxford/New York, p556-559
Daucher JN, Mandell J, Lebowitz RL Urinary tract infection in infants in spite of prenatal diagnosis of hydronephrosis Pediatr Radiol 22:401-4
Petrikovsky BM, Cvomo MI, Schneder EP, Wyse W, Cohen HL, Lesser M Isolated fetal hydronephrosis: beware of the effect of bladder filling Prenatal Diagnosis 15:827-829
Chervenak FA, Issacson GC, Campbell S In: Ultrasound in Obstetrics and Gynecology Vol I Little, Brown & Co: Boston, p970-980