Marked maternal ureteropelvic obstruction mimicking a large ovarian mass at 20 weeks' gestation

Physiologic dilatation of the maternal urinary collecting system occurs in more than 80% of gravidas by mid gestation. Caliceal and ureteral dilation are more common on the right, and, similarly, the degree of caliceal dilatation is more pronounced on the right (15 versus 5 mm). Although most women with hydronephrosis during pregnancy are asymptomatic, a few may have clinical signs and symptoms related to the upper urinary system. When left-sided hydronephrosis is greater than the right, particularly if the patient is symptomatic, the likelihood of a pathologic condition, especially obstructive, should be considered. We present an unusual case in which a large left upper abdominal cystic structure depicted sonographically at mid gestation, initially considered consistent with an ovarian cyst, resulted from marked maternal ureteropelvic obstruction and required nephrostomy.