Urorectal septum malformation sequence: prenatal sonographic diagnosis in two sets of discordant twins.

Urorectal septum malformation sequence (URSMS) is a rare congenital malformation, which includes ambiguous genitalia, a phallus-like structure, imperforate anus, bladder, vaginal and rectal fistulas and Müllerian duct defects. We report two cases of prenatally diagnosed URSMS, both occurring in two sets of discordant twins. To the best of our knowledge, this is the first antenatal description of such an anomaly. The first fetus, one of a set of monochorionic, monoamniotic twins was detected sonographically at 21 weeks of gestation due to an enlarged phallus-like formation. The second fetus, one of dichorionic, diamniotic twins, was suspected of having an abnormally enlarged rectum at 13 weeks of gestation. The diagnosis of URSMS was established at 29 weeks of gestation by showing abnormal female external genitalia, with a dilated bowel that contained echogenic foci due to enterolithiasis. The diagnosis of both cases was confirmed postnatally. Sonographic findings and differential diagnosis are presented.