description

a fetus-in-fetu is an encapsulated, pedunculated vertebrate tumor. it represents a malformed, monochorionic diamniotic parasitic twin included in a host (or autosite) twin. characteristically the fetus-in-fetu complex will be composed of a fibrous membrane (equivalent to the chorioamniotic complex) that contains some fluid (equivalent to the amniotic fluid) and a fetus suspended by a cord or pedicle. the presence of rudimentary spinal architecture is used to differentiate a fetus-in-fetu from a teratoma, since teratomas are not supposed to develop through the primitive streak stage (12-15 days). this aspect has been considered too stringent by many authors who regard rudimentary body architecture (metameric segmentation, craniocaudal and lateral differentiation, body coelom) as an equivalent criterion. although teratomas can achieve striking degrees of differentiation by the inductive effect of adjacent tissues on one another, they do not present the criteria mentioned above. they also occur predominantly in the lower abdomen, not the upper retroperitoneum. teratomas also have a definite malignant potential, a feature that has not been reported in fetus-in-fetu. however, the coexistence of a fetus-in-fetu and a teratoma as well as the occurrence of a teratoma 14 years after removal of a twin fetus-in-fetu has been reported, supporting the older hypothesis of a continuum between twin and teratoma. cases of sacrococcygeal fetus-in-fetu should probably be regarded and treated as teratoma, because of the high incidence of teratoma in this region. ectopic testicles have a higher incidence of germ cell tumors, and the differentiation between fetus-in-fetu and teratoma is particularly important. some have argued that fetus-in-fetu should be considered as a teratoma since they do not evolve into lithopedion like fetuses of abdominocyesis. that argument is probably not valid since in abdominocyesis the antigen complements of the host and fetus are different, contrasting with fetus-in-fetu.