Surgeons in Peru will operate on a 3-year-old boy to remove the body of a “parasitic twin” growing inside of the toddler’s stomach on Tuesday.

Isbac Pacunda has in his stomach a partially formed 9-inch-long fetus that weighs a pound and a half. Physicians said that Pacunda’s would-be twin has eyes, bones, and even hair on the cranium, but had not yet developed a brain, lungs, heart or intestines, Dr. Carlos Astocondor, a plastic surgeon at the Las Mercedes Hospital in Chiclayo told Associated Press.

Pacunda’s condition is called fetus-in-fetu, a rare condition when a fetus absorbs his twin sibling inside the womb when an egg fails to fully separate, and happens approximately once out of every 500,000 live births, Astocondor said.

The fetus-in-fetu condition, although rare, is not unheard of. In 2008, doctors had to remove a 2-inch embryo from the body of a 9-year-old girl in Greece, and in 2006 doctors discovered a half-formed twin with limbs, genitalia, hair and even fingernails in the protruding stomach of a 36-year-old Indian man.

Identical twins formed by a fertilized egg that splits in half, but conjoined twins or fetus-in-fetu siblings are made when the egg fails to fully separate.

Dr. Jonathan Fanaroff, a neonatologist at Rainbow Babies and Children’s Hospital in Cleveland told the Daily Mail that some conjoined twins can survive as “parasites” relying on the body of the other for blood supply and organ function, but not when one twin engulfs the other, like in Pacunda’s case, according to ABC News.

Fanaroff said that the surgical operation to remove the 3-year-old’s parasitic twin was probably far easier than attempting to separate two living conjoined twins.