Fetal pyelectasis

Fetal pyelectasis (FP) refers to a prominence/dilatation of the renal pelvis in utero. It is at the mild end of the fetal hydronephrosis spectrum.


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 trimeter 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

Radiographic features


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.

Treatment and prognosis

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. 

See also

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Ultrasound - obstetric

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