A fetus can get trapped inside of its twin

Fetus in fetu (or Foetus in foetu) describes an extremely rare abnormality that involves a fetus getting trapped inside of its twin. It continues to survive as a parasite even past birth by forming an umbilical cord-like structure that leeches its twin's blood supply until it grows so large that it starts to harm the host, at which point doctors usually intervene. Invariably the parasitic fetus is anencephalic (without a brain) and lacks internal organs, and as such is unable to survive on its own, though it may have almost human (albeit underdeveloped and bizarre) features such as limbs, digits, hair, nails and teeth. Fetus in fetu is such a rare condition that only some 91 cases worldwide have ever been reported. Fetus in fetu happens very early in a twin pregnancy, when one fetus wraps around and envelops the other. The dominant fetus grows, while the fetus that would have been its twin lives on throughout the pregnancy, feeding off its host twin like a kind of parasite. Usually, both twins die before birth from the strain of sharing a placenta. Sometimes, however, the host twin survives and is delivered.