Foetus in foetu is a developmental abnormality in which a mass of tissue resembling a foetus forms inside the body. An early example of the phenomenon was described in 1808 by George William Young. There are two theories of origin concerning "foetus in foetu". One theory is that the mass begins as a normal foetus but becomes enveloped inside its twin. The other theory is that the mass is a highly developed teratoma. "Foetus in foetu" is estimated to occur in 1 in 500,000 live births.
A foetus in foetu can be considered alive, but only in the sense that its component tissues have not yet died or been eliminated. Thus, the life of a foetus in foetu is akin to that of a tumor in that its cells remain viable by way of normal metabolic activity. However, without the gestational conditions in utero with the amnion and placenta, a foetus in foetu can develop into, at best, an especially well differentiated teratoma; or, at worst, a high-grademetastaticteratocarcinoma. In terms of physical maturation, its organs have a working blood supply from the host, but all cases of foetus in foetu present critical defects, such as no functional brain, heart, lungs, gastrointestinal tract, or urinary tract. Accordingly, while a foetus in foetu can share select morphological features with a normal foetus, it has no prospect of any life outside of the host twin. Moreover, it poses clear threats to the life of the host twin on whom its own life depends.
Theories of development
There are two main theories about the development of foetus in foetu.
Teratoma theory
Foetus in foetu may be a very highly differentiated form of dermoid cyst, itself a highly differentiated form of mature teratoma.
Foetus in foetu may be a parasitic twinfetus growing within its host twin. Very early in a monozygotictwin pregnancy, in which both foetuses share a common placenta, one foetus wraps around and envelops the other. The enveloped twin becomes a parasite, in that its survival depends on the survival of the host twin, by drawing on the host twin's blood supply. The parasitic twin is anencephalic and lacks some internal organs, and as such is unable to survive on its own. As the host twin has to "feed" the enveloped twin from the nutrients received over a single umbilical cord, they usually die before birth.
