Pop-off mechanisms in fetal megacystis: extravasation, umbilical cord cyst, ureterocele and megaureter.

Objective: To analyze comprehensively the incidence, antenatal ultrasound characteristics and prognostic implications of antenatal pop-off mechanisms of the fetal urinary system in pregnancies with suspected fetal megacystis.

Methods: This was a retrospective multicenter study of pregnancies with suspected fetal megacystis conducted across all academic hospitals in The Netherlands. Three antenatal pop-off mechanisms were identified: presence of an umbilical cord cyst (UCC), extravasation of urine into the intraperitoneal space (ascites) or perirenal subcapsular (urinoma), and megaureter/ureterocele. Cases that exhibited two different pop-off mechanisms, underwent vesicoamniotic shunt placement or had unclear information regarding shunt placement were excluded. We compared the antenatal ultrasound characteristics and outcomes among pregnancies with UCC, those with extravasation, those with megaureter/ureterocele and those without a pop-off mechanism. Logistic regression analysis was used to evaluate the association of pop-off mechanisms with antenatal characteristics and postnatal outcomes.

Results: Among 543 fetuses with suspected megacystis, 76% exhibited no pop-off mechanism, 7% presented with UCC only, 9% presented with extravasation only, 7% presented with a megaureter/ureterocele only and 1% presented with two pop-off mechanisms. Following exclusions, 511 cases were included in the analysis. The identification of UCC (n = 39) was associated with early-onset megacystis (odds ratio (OR), 4.2 (95% CI, 1.9-9.1); P < 0.001), severe megacystis (OR 2.3 (95% CI, 1.1-5.0); P = 0.033), normal amniotic fluid index (AFI) (OR, 3.3 (95% CI, 1.3-8.2); P = 0.011) and additional associated anomaly (OR, 3.3 (95% CI, 1.7-6.4); P < 0.001), and thus with the highest prevalence of complex diagnosis (66%), primarily represented by anorectal malformation. Extravasation (n = 42) was associated with severe megacystis (OR, 2.4 (95% CI, 1.1-5.4); P = 0.030), abnormal AFI (OR, 2.8 (95% CI, 1.2-6.8); P = 0.022), the keyhole sign (OR, 2.5 (95% CI, 1.1-5.8); P = 0.033) and additional associated anomaly (OR, 2.1 (95% CI, 1.1-4.1); P = 0.026). Megaureter/ureterocele (n = 36) was associated with late-onset megacystis (OR, 4.0 (95% CI, 1.6-9.7); P = 0.003), a thickened bladder wall during pregnancy (OR, 6.6 (95% CI, 1.9-23.1); P = 0.003) and the lowest prevalence of additional associated anomaly (22%). Intrauterine fetal demise was most prevalent in fetuses with UCC (15%), while termination of pregnancy and non-survivors were most common in cases with extravasation (50% and 17%, respectively). The majority of fetuses with megacystis associated with megaureter/ureterocele were still alive during follow-up (72%) and the odds of survival were the highest for this group (OR, 2.7 (95% CI, 1.3-5.7); P = 0.010).

Conclusions: Antenatal pop-off mechanisms may alleviate high intraluminal pressure within the fetal urinary tract. Each mechanism leads to a different antenatal clinical picture and outcome, which may explain partially the heterogeneity of outcomes in fetuses with megacystis. Therefore, understanding the implications of these mechanisms and their antenatal characteristics could guide antenatal counseling and management of fetal megacystis. © 2025 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.