Fetus in fetu--a mystery in medicine.

Fetus in Fetu (FIF) is a rare condition where a monozygotic diamnionic parasitic twin is incorporated into the body of its fellow twin and grows inside it. FIF is differentiated from teratoma by the presence of vertebral column. An eight year old girl presented with an abdominal swelling which by X-ray, ultrasonography and CT scan revealed a fetiform mass containing long bones and vertebral bodies surrounded by soft tissue situated on right lumber region. On laparotomy, a retroperitoneal mass resembling a fetus of 585 gm was removed. It had a trunk and four limbs with fingers and toes, umbilical stump, intestinal loops and abundant scalp hairs but was devoid of brain and heart. Histology showed various well-differentiated tissues in respective sites. FIF is a mystery in reproduction and it is scarce in literature in such well-developed stage.