Fetal pyelectasis

Fetal pyelectasis refers to a prominence of the renal pelvis in utero that is a relatively common finding, which in the majority of cases resolves spontaneously. 

Please refer to the article on fetal hydronephrosis for a continued discussion on this matter. 

Although there is an overlap of definition between pyelectasis and hydronephrosis, the former has been widely used instead of mild hydronephrosis given that the vast majority of the cases represent only a physiological incidental finding that resolves spontaneously, while the latter tends to be reserved for cases where a pathological obstruction is suspected. 

Fetal pyelectasis can be a relatively common finding on antenatal ultrasounds, often detected at the routine 2nd-trimester morphology scan. There is a recognised male predilection. The estimated prevalence is at ~2% of routine second trimester scans 13.

Pyelectasis can result from a number of factors. In the majority of cases, it is physiological and resolves spontaneously. However, it may also herald the presence or evolution of renal tract pathology, such as:

  • trisomy 21: presence and strength of this association is currently under debate and at best considered very mild

Fetal pyelectasis is assessed as an AP measurement of the renal pelves on an axial plane ultrasound image. It is considered present if measurements are above the levels stated below:

  • >4-4.5 mm at 18-20 weeks (i.e. the routine 2nd trimester)
  • >5 mm at ~20-29 weeks
  • >6 mm at 32 weeks
  • persistent fetal pyelectasis: >7 mm in the 3rd trimester 8

Fetal pyelectasis can also be affected by maternal hydration: physiological fetal pyelectasis 5.

The vast majority of cases (~96%) with mild pyelectasis in the second trimester resolve, either during pregnancy or in the early postpartum period. The risk of postnatal renal pathology is increased with: 

  • increasing degree of pelvic dilatation
  • in utero progression
  • bilateral involvement

Antenatally detected renal pelvic dilatation, especially in isolation, is considered a weak predictor of vesicoureteric reflux 7 although postnatal sonographic evaluation is often recommended.

Some advocate a repeat prenatal scan at 30-40 weeks gestation for fetuses if ≥6 mm of renal pelvic dilatation is detected prior to 28 weeks as well as postnatal follow-up for persistent pyelectasis 10.

Postnatally, most cases with pyelectasis resolve spontaneously in the first year of life, and invasive procedures are not required. ​

Ultrasound - obstetric
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Article information

rID: 13405
Section: Gamuts
Tag: cases
Synonyms or Alternate Spellings:
  • Fetal renal pelvic dilatation
  • Fetal renal pelvic dilatation (RPD)
  • Mild fetal pyelectasis
  • Fetal pyelectasis (FP)

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