Both the definition and the significance of fetal renal dilatation are controversial topics in the obstetric and ultrasound literature. The following is our approach to this difficult topic.


Hydronephrosis is present if any one of three conditions exists:

  1. The AP renal pelvis diameter measures 1 cm
  2. Caliectasis (distention of the calices) is present
  3. The AP renal pelvis measures > 1/2 of the AP diameter of the kidney

Anything less than this is pyelectasis (see below). During the fetal survey, we make particular note of the bladder and amniotic fluid volume.

The following recommendations are made for the finding of hydronephrosis:

  1. if associated with other findings, genetic counseling (due to the association with aneuploidy)
  2. one follow-up scan in the third trimester to assess fluid, and any progression
  3. renal sonogram after the baby is born (at least a week afterwards, to allow for rehydration).


Pyelectasis is defined as central renal dilatation which is less than hydronephrosis. In order to be mentioned in our report it must be:

  1. > 4 mm in the second trimester (up to 28 weeks) OR >7 mm in the third trimester (after 28 weeks)
    This is mentioned in the body of the report, but do not recommend follow-up in utero unless it is bilateral. The baby should have a renal sonogram (at least a week after birth, to allow for rehydration).
  2. ****Note that lesser degrees of dilation are treated as incidental findings, For example, 3 mm of dilation in the second trimester or 6 mm in the third trimester should be mentioned in the body of the report (just like we mention simple renal cysts in adult renal scans), with the phrase, "this degree of dilation is not felt to be clinically significant."


  • Wickstrom E, Maizels M, Sabbagha RE, Tamura RK, Cohen LC, Pergament E. Isolated fetal pyelectasis: assessment of risk for postnatal uropathy and Down syndrome. Ultrasound Obstet Gynecol 8 (1996) 236-240.
  • Persutte WH, Koyle M, Lenke RR, Klas J, Ryan C, Hobbins JC. Mild pyelectasis ascertained with prenatal ultrasonography is pediatrically significant. Ultrasound Obstet Gynecol 10 (1997) 12-18.
  • Anderson N, Clautice-Engle T, Allan R, Abbott G, Wells JE. Detection of obstructive uropathy in the fetus: predictive value of sonographic measurements of renal pelvic diameter at various gestational ages. AJR 1995:164:719-723.
  • Corteville JE, Gray DL, Crane JP. Congenital hydronephrosis: correlation of fetal ultrasonographic findings with infant outcome. Am J Obstet Gynecol 1991;165:384-8.