Fetus in fetu 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.