Fetus in fetu: report of three cases and review of the literature

Fetus in fetu, or parasitic twin, is an uncommon condition in which a malformed fetus grows within the body of its twin. It is almost always detected as an abdominal mass in infancy: 41 cases have been reported since 1806 and 89% of the patients were less than 1 year old. We have reviewed the literature and report three new cases from our own experience. All patients available were considered from the point of view of their clinical findings, preoperative diagnosis, surgical and anatomic-histological aspects. We also suggest the possibility of including those cases otherwise defined as teratomas with a high grade of structural complexity (limbs, well formed organs, or parts of systems) in this pathological group even if they do not show a real vertebral column according to Willis' definition.