Fetal Pyelectasis (Pelviectasis)

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Fetal Pyelectasis (Pelviectasis)

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The tubes that carry urine from the kidneys to the bladder are called the ureters. The upper part of the ureter that funnels urine formed by the kidneys to the lower ureter 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 likelihood of an underlying abnormality correlates with the severity of the hydronephrosis.

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 and may be due to an immature ureter or folds of tissue in the ureter.The most common cause of severe pyelectasis or hydronephrosis is blockage of the ureter where at the pelvis, uretero-pelvic junction obstruction (UPJO) [1 in figure]

UPJO may be caused by abnormal functioning of the smooth muscle of the pelvis and ureter, or a physical blockage of the ureter caused by vessels lying across the ureter. Vesicoureteric reflux (backward flow of urine from the bladder) is another relatively common cause of severe pyelectasis or hydronephrosis [2 in figure]. Less common causes of hydronephrosis include urethral atresia, ectopic ureteroceles, duplication of the collecting system, cloacal malformation., posterior urethral valves (PUVs) [3 in figure ] ,and megacystis-microcolon-intestinal-hypoperistalsis syndrome. (MMIHS). With PUVs and MMIHS an enlarged bladder is seen in addition to hydronephrosis in both kidneys.

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 pelvis(es) measure ≥7 mm at 32 weeks or later of gestation, then the infant should be evaluated after delivery by the pediatric provider to determine if pediatric urology or nephrology followup is needed

By Mark Curran, MD FACOG Updated 10/28/2021

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