Antenatal retroperitoneal mass suspected as teratoma: Postnatal diagnosis of fetus in fetu

Fetus in fetu (FIF) is a rare congenital anomaly in which a malformed parasitic twin is enclosed within its host, most commonly in the retroperitoneum. Antenatal detection is uncommon and often mimics teratoma on prenatal imaging. We report a neonate with an antenatally detected retroperitoneal mass initially suspected to be a teratoma on ultrasonography and fetal magnetic resonance imaging. Postnatal computed tomography demonstrated organized vertebral and limb-like skeletal elements, confirming FIF. Complete surgical excision was curative. This case underscores the pivotal role of multimodality imaging in distinguishing FIF from other neonatal retroperitoneal masses.