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Practical obstetrics info for your women's healthcare practice

Renal Pyelectasis on Prenatal Ultrasound – Next Steps?

Renal pyelectasis literally means “pelvis dilation” of the kidney, and is defined as an anteroposterior diameter of the renal pelvis of ≥ 4mm up to 20 weeks of gestation.


When seen in isolation, with no other fetal abnormalities

  • If aneuploidy screening has not yet been done, then screening or diagnostic testing should be offered
  • If patient opts for aneuploidy testing and result is negative, no further evaluation is required
    • Counsel patients that screening tests are not diagnostic and therefore there is still residual risk for chromosomal anomalies
  • If aneuploidy screen result is positive, refer for genetic counseling and consideration of diagnostic testing options
  • Ultrasound at 32 weeks of gestation is suggested to rule out persistent pyelectasis and possible obstruction of the urinary tract
    • If the renal pelvis is > 7 mm at 32 weeks, then post-natal follow up is suggested

If other anomalies are detected

  • ACOG guidance recommends offering invasive testing using microarray in the setting of fetal structural anomalies seen on prenatal ultrasound


Renal pyelectasis is reported in 0.6-4.5% of fetuses in the second trimester. It is most commonly a transient physiologic state in which the renal pelvis, the structure that is essentially the funnel for urine exiting the kidney into the ureter, is dilated and measures larger than what is considered normal for a particular gestational age. The presence of renal pyelectasis as an isolated finding, in the setting of a negative aneuploidy screen, is not itself an indication for invasive testing; however, ACOG does give all women the option (both high and low risk) for invasive testing.  Note, if the renal pelvis is still dilated in the third trimester, the urinary tract may require attention following delivery to ensure normal kidney function and prevent infection in the newborn.


  • Normal renal pelvis measurement is up to 1cm throughout gestation
    • Values above 1cm are almost always pathological
  • Resolution of dilation based on second trimester measurement
    • Between 4 and 7 to 8 mm: 80% resolution
    • >9 mm: <15%
  • More commonly observed in male fetuses
  • A targeted ultrasound is essential to look for other fetal anatomic anomalies
  • Note: ACOG guidance recommends offering invasive, diagnostic testing for all pregnant women regardless of age and even in the presence of a normal anatomy scan
    • Offering invasive testing using microarray in the setting of fetal structural anomalies on prenatal ultrasound is recommended

Learn More – Primary Sources:

Multidisciplinary consensus on the classification of prenatal and postnatal urinary tract dilation (UTD classification system)

ACOG Practice Bulletin No. 162: Prenatal Diagnostic Testing for Genetic Disorders 

ACOG Practice Bulletin No. 226: Screening for Fetal Chromosomal Abnormalities

Identification of copy number variations among fetuses with ultrasound soft markers using next-generation sequencing

Locate a genetic counselor or genetics services:

Genetic Services Locator-ACMG

Genetic Services Locator-NSGC

Genetic Services Locator-CAGC