Doctors will remove the partial body of a twin from 3-year-old Isbac Pacunda's stomach. Image credit: Karel Navarro/AP Photo

Doctors in Peru said they would operate today on a 3-year-old boy to remove the body of his would-be twin.

Isbac Pacunda has the body of his twin inside his stomach - bones, eyes and even hair on the cranium. Dr. Carlos Astocondor, a plastic surgeon at the Las Mercedes Hospital in Chiclayo, told the Associated Press that the partially formed fetus weighs about a pound and a half and is 9  inches long. He and a team of 12 doctors will surgically remove the tissue from the boy's stomach today.

The condition is called fetus-in-fetu and happens in about one  out of every 500,000 live births, Astocondor said. Although rare, these cases are not unheard of. In 2008, doctors removed a 2-inch embryo from the body of a 9-year-old girl in Greece. In 2006, doctors found a half-formed twin with limbs, genitalia, hair - even fingernails - in the swollen stomach of a 36-year-old Indian man.

The case spotlights the biological precision needed for twins to develop in the womb. Identical twins form when an egg splits in half right after fertilization. If the egg fails to fully separate, conjoined twins will form, sharing some portion of their bodies.

Dr. Jonathan Fanaroff, a neonatologist at Rainbow Babies and Children's Hospital in Cleveland, told ABC News that some conjoined twins can live as "parasites," relying on the body of the other for a blood supply and organ function. But for fetus-in-fetu, the body of one twin envelops the other during development.

"In this kind of situation, because it was inside the other boy, it wasn't able to survive," Fanaroff said.

Surgery to separate live, conjoined twins is extremely difficult and fraught with ethical complications. Fanaroff said the removal of Pacunda's twin will likely be much easier.