Fetal Pyelectasis (Pelviectasis)

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Fetal Pyelectasis (Pelviectasis)

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The upper part of the ureter that funnels urine formed by the kidneys to the lower ureter and bladder is called the renal pelvis. Enlargement of the renal pelvis from 4 to 10 millimeters in diameter is called fetal pyelectasis or renal pelvic dilatation (RPD). .Enlargement of the renal pelvis to greater than 10 mm is called severe pyelectasis or hydronephrosis. The illustration at right shows a kidney with a normal renal pelvis on the left compared to a kidney with pyelectasis on the right.

The most common causes of severe pyelectasis or hydronephrosis are a blockage of the ureter (the tube that carries urine from the kidney to the bladder) where it joins the kidney [1 in figure] and backward flow of urine from the bladder (vesicoureteric reflux) [2 in figure]. Less common causes of hydronephrosis include posterior urethral valves [3 in figure ] , urethral atresia, ectopic ureteroceles, duplication of the collecting system, megacystis-microcolon-intestinal-hypoperistalsis syndrome, prune belly syndrome, and cloacal malformation.

Pyelectasis is detected in 0.6%–4.5% of fetuses in the second trimester, and is seen more commonly in male fetuses. Most mild cases resolve before birth or within a few months after delivery. A fetus with severe pyelectasis or hydronephrosis is at increased risk of developing problems such as low amniotic fluid and kidney disease. In addition pyelectasis has a weak association with chromosome abnormalities, in particular Down syndrome . The severity of isolated pyelectasis does not appear to increase the risk for chromosome abnormality .A detailed ultrasound of the fetus is recommended to look for fetal abnormalities or other ultrasound markers associated with Down syndrome .

The American College of Obstetricians and Gynecologists recommends if more than one marker for aneuploidy (abnormal number of chromosomes) is identified, then genetic counseling, maternal–fetal medicine consultation, or both are recommended. If pyelectasis is isolated and aneuploidy screening has not been performed, the patient should be counseled regarding the risk of aneuploidy associated with the finding , and cell-free DNA , quad screen testing, or amniocentesis should be offered.

If pyelectasis is an isolated finding and aneuploidy testing (serum screening or cell free fetal DNA) is performed and is low-risk, then no further risk assessment is needed. Repeat ultrasound at 32 weeks is recommended to evaluate for persistence or worsening of pyelectasis . If the renal pelvises measure ≥7 mm at 32 weeks of gestation, then the infant should be evaluated after delivery.

By Mark Curran, MD FACOG Updated 3/28/2021

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