A young woman recently gave birth to premature twin boys in Xi'an, northwestern China's Shanxi province. But to everyone’s surprise, there was a third baby in the belly of one of the twins.

Doctors checked a mass in the boy's lower abdomen. It was about 6 to 7 centimeters in diameter, and an operation was required to remove it. 

The operation on a 2-week-old, 2-kilo baby was a severe test both for the boy and the doctors. After four hours, the mass was successfully removed. 

The mass looked like a rare parasitic fetus; bones and fingernails were discovered within it after doctors cut it open, said Gao Ya, a professor and Vice-Dean of the Second Affiliated Hospital of Xi'an Jiaotong University 

The odds of this fetus-in-fetus condition are only one in 500,000 worldwide. The twin that hosted the fetus has been slowly restored to health after the surgery.