Fetus in fetu: a case report and review of the literature.

A cystic tumor was first found in the abdomen of a Taiwanese girl at the age of 9 months. The tumor gradually increased in size and caused no symptoms until the girl was 7 years of age, when surgical removal was carried out. Pathological examination disclosed two "fetuses" within the fibrous sac. The pathogenesis of fetus in fetu and its differentiation from retroperitoneal teratoma are still controversial issues. Further work needs to be done to elucidate whether it is a natural progression of the twinning process to teratoma or the inclusion of a monozygotic diamniotic twin within the bearer.