Hydronephrosis Secondary to an Ectopic Decidual Reaction in the Urinary Bladder

A 28-year-old pregnant woman at 19 weeks gestation presented with dysuria as well as lower abdominal and left flank pain. Imaging revealed left-sided hydronephrosis and a mass invading the posterior bladder wall. Management included placement of a left nephrostomy tube and transurethral resection of ~25% of the mass. Microscopy showed an ectopic decidual reaction within the muscularis propria. The patient improved symptomatically and continued prenatal care. Complete resolution of her ureteral obstruction was demonstrated during the postpartum period. Ectopic decidual reactions involving the urinary bladder are extremely rare, and ureteral obstruction secondary to this phenomenon has not yet been reported.