Fetus in fetu (or Foetus in foeto) 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.

And the abcnews article:

It is an extremely rare abnormality that involves a fetus getting trapped inside of its twin. The trapped fetus can 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.

I wonder who came first, or of they are both taking from somewhere else. The last edit on the wikipedia page is from July of 2006. I wonder if the abcnews article is recycled from something earlier as well.