Fetus in fetu

Fetus in fetu, also spelled foetus in foetu, is a rare developmental abnormality in which a mass of tissue resembling a fetus forms inside the body of a true fetus. An early example of the phenomenon was described in 1808 by George William Young. Two hypotheses are offered for its origin. In one, the mass begins as a normal fetus that becomes enveloped inside its twin early in development. In the other, the mass is a highly developed teratoma, a tumor of mixed tissue types. The condition is estimated to occur in about 1 in 500,000 live births, or 0.0002 percent. The article also notes that the mass is not expected to survive outside its host twin.