An 11-year-old girl had a life-changing procedure recently when doctors discovered that she had a third arm emerging from her back.

The girl was afflicted with an extremely rare condition called fetus in fetu. The condition affects just one in 5 million births. Researchers believe that the arm belonged to an unborn twin sister who was growing inside of her. The girl was the second child in a family that had not previously reported twin births or birth abnormalities.

As rare as the situation is in itself, this unnamed girl's particular case is extremely unheard of. Normally, when fetus in fetu is discovered, it is because of a growth on the stomach; that occurs in 80 percent of cases. Such growths have also been discovered on the skull, sacrum, scrotum, and mouth. Her case was the first time organs from a twin had been discovered growing from a person's back.