Maternal horseshoe kidney presenting as an incidental retroperitoneal mass at mid-trimester sonography.

The maternal retroperitoneum is seldom visualized during routine prenatal sonography. Although uncommon, maternal retroperitoneal masses complicating pregnancy have been reported. We report an unusual case in which an incidental sonographic finding of a large, well-demarcated, midline, heterogeneous, maternal retroperitoneal mass indenting the posterior aspect of the uterus at 21 weeks' gestation, later following MR imaging proved to be the inferior isthmus of previously undiagnosed maternal horseshoe kidney. This case demonstrates an additional benign etiology of a maternal retroperitoneal mass in pregnancy and emphasizes the characteristic sonographic findings associated with this condition and the importance of visualizing the maternal retroperitoneum at midtrimester scanning.