Origin, formation mechanism and prenatal diagnosis of the intracranial Fetus-in-fetu. Review of the literature

Fetus-in-fetu (FIF) is an extremely rare congenital condition, in which a fetus-like structure is found in the body of its twin. Its embryopathogenesis has not yet been established. Many investigators believe FIF represents a highly organized teratoma. More recently, the FIF has been defined as any structure in which the fetal-form is in a very high development of organogenesis with the presence of a vertebral axis. In literature less than 200 cases have been reported, mostly intra-abdominal and only 17 have been found intra-cranially. The prenatal ultrasonographic diagnosis of intracranial FIF has been suspected in 8 cases, but only one has been correctly diagnosed because a heartbeat that was of different rate to its host twin was found in the fetal-form mass.