Fetus in fetu (FIF) is a rare disorder that can be a challenge to diagnose.
Features:
(1) a mass of fetal tissue which is usually (but not always) formed around a vertebral axis and which may have recognizable or vestigial organs and/or limbs
(2) usually located in the peritoneum cavity, presenting as an intra-abdominal mass
(3) imaging studies may show cystic areas, fat and/or calcifications
Theories of origin:
(1) mature fetiform teratoma
(2) diamniotic monochrorionic twin gestation with one fetus being incorporated within its twin prior to fusion of the lateral body walls
Comparing the cytogenetics of the mass and the host may be helpful.
Differential diagnosis:
(1) meconium peritonitis
(2) intra-abdominal tumor, including a true teratoma
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