Fetus in fetu
（重定向自Foetus in foetu）
![Anteroposterior abdominal radiograph shows a soft-tissue mass in the right hemiabdomen. The mass contains calcified osseous-appearing structures of varying sizes and shapes.](/Images/godic/202501/13/Fetus_in_fetu_radiograph5541.jpg")
![The postoperative specimen shows a fairly well developed fetus lying on its back, with rudimentary digits.](/Images/godic/202501/13/Fetus_in_fetu_after_operation5541.jpg")
![Computed Tomography scan of the patient's abdomen reveals a large retroperitoneal soft-tissue mass. There are long hyperdense opacities that resemble fetal bones.](/Images/godic/202501/13/Fetus_in_fetu_Computed_Tomography_scan5541.jpg")
Fetus in fetu (or foetus in foetu) is a developmental abnormality in which a mass of tissue resembling a fetus forms inside the body. There are two theories of origin concerning "fetus in fetu". One theory is that the mass begins as a normal fetus but becomes enveloped inside its twin. The other theory is that the mass is a highly developed teratoma. "Fetus in fetu" is estimated to occur in 1 in 500,000 live births.