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.

Usually, both twins die before birth from the strain of sharing a placenta, but worldwide there have been 91 living cases reported after birth.

The in fetu twin continues to survive as a parasite inside its twin, 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.

The parasitic fetus is almost always 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.