Doctors in Peru have found a “parasitic twin” in the stomach of a 3-year-old boy, and plan to surgically remove the tissue Monday.Dr. Carlos Astocondor of the medical team at Las Mercedes Hospital in the northern port of Chiclayo says the condition occurs in about one of every 500,000 live births.

Astocondor will lead a team of 12 other physicians to remove the excess tissue from Isbac Pacunda in an operation that will take several hours.

He says the partially formed fetus weighs a pound and a half and is nine inches long.

Astocondor says the brain, heart, lungs and intestines never developed after the fetus was absorbed by the other fetus inside the mother’s womb. He says it has some hair on the cranium, eyes and some bones.

Cases like these, where the twin was absorbed and not able to live outside its sibling, are commondly referred to as “Fetus in fetu.”