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.

He says the partially formed fetus weighs a pound and a half (700 grams) and is nine inches (25 centimeters) 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.