What is "fetus in fetu"

n. (context medicine English) A fetus growing inside the body of another fetus, either from an unsuccessful twin pregnancy, or as a tumor-like growth.

Fetus in fetu (or foetus in foetu) is a developmental abnormality in which a mass of tissue resembling a fetus forms inside the body. There are two theories of origin concerning "fetus in fetu". One theory is that the mass begins as a normal fetus but becomes enveloped inside its twin. The other theory is that the mass is a highly developed teratoma. "Fetus in fetu" is estimated to occur in 1 in 500,000 live births.