Retroperitoneal fetus-in-fetu: another supportive evidence for twinning theory

An 8-year-old boy with large abdominal lump causing upper abdominal fullness is presented. Clinically it mimicked a trichobezoar because of the restricted side-to-side mobility of the lump. The lump, however, had calcification and radiological features of fetus-in-fetu (FIF). At laparotomy, a fetiform mass covered with fatty tissue was delivered out of a retroperitoneally located sac (in the bed of stomach). The fetiform mass had an identifiable upper limb bearing digits, one of them also bearing nail. Though histopathological examination did not reveal the presence of vertebral bodies, an advanced differentiation of the bony ends, bearing cartilage qualified the mass as FIF. There was family history of twinning with the elder siblings being dizygotic twins. The origin and diagnostic criteria of FIF are discussed and reference is made to a high likelihood of a twinning as the preceding event in this case.