Dr Carlos Astocondor of the medical team at Las Mercedes Hospital in the northern port of Chiclayo said the condition occurs in about one of every 500,000 live births.
He said the partially formed foetus weighs a pound and a half and is nine inches long.
Dr Astocondor said the brain, heart, lungs and intestines never developed after the foetus was absorbed by the other foetus inside the mother's womb.
He said it has some hair on the cranium, eyes and some bones.
The surgery is set to be carried out on Isbac Pacunda today.