... is a developmental abnormality: a mass of tissue inside the body that more or less resembles a fetus. 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.
Maybe splitting hairs (or something) but not quite the same thing as parasitic. Get your freaks straight. And, see? I'm not being entirely inaccurate when I call your unborn a tumor.