Prenatal diagnosis of fetus in fetu with spontaneous movement of the parasitic twin

Fetus in fetu (FIF) is a rare congenital anomaly in which a malformed parasitic twin is enclosed within its host. A right-sided fetal intra-abdominal mass was incidentally detected during routine antenatal ultrasonography at 26 weeks of gestation in a 40-year-old gravida 4, para 2 woman. Ultrasonography at 27 weeks showed a right-sided, well-encapsulated solid and cystic mass with skeletal elements and gentle arterial flow toward the solid component. Real-time assessment revealed a brief spontaneous adduction-like inward movement of the parasitic lower limb, suggestive of FIF rather than teratoma. At 35 weeks, fetal magnetic resonance imaging demonstrated a well-encapsulated right intra-abdominal mass with internal low-signal-intensity structures corresponding to the bony elements seen on ultrasonography. Following birth, the mass was surgically excised and histopathology confirmed FIF. This case illustrates the diagnostic value of detailed dynamic imaging for recognizing FIF and may contribute to the understanding of its pathophysiology.