Call it one in two million. Anias and Jadon McDonald, twins conjoined at the head, are undergoing separation surgery today at 13 months old, as reported by CNN.

Twins joined at the head, called craniopagus twins, occur in 1 out of every 2.5 million births. “About 40% of the twins are stillborn and another one-third die within 24 hours of their birth. Studies have shown 80% of twins joined at the head die of medical complications by the age of 2 if not separated,” the report stated.

Parents of the twins, Nicole and Christian McDonald, gave exclusive access to CNN. After months of deliberation, the parents decided that separation was best for the twins.

"I can't wait to see them as two separate little boys," said Christian. "That's what excites me the most. I really just want to know my boys."

The surgery is being performed at the Children's Hospital at Montefiore Medical Center in The Bronx, New York. The surgeon leading the operation is Dr. James Goodrich, considered by many to be the world's leading neurosurgeon for conjoined twins.

"This [surgery] about as complicated as it gets," Goodrich says. But he affirms, "we've got it down to a fine art.”

Dr. Oren Tepper, a plastic surgeon who will reconstruct the boys' skulls and stitch their heads closed, will join Goodrich. "This is a once-in-a-career case," he said.

The possibility of death or long-term brain damage for one or both boys frighten Nicole and Christian. If the twins survive the surgery, they will have to undergo months of intense recuperation and rehabilitation.

Yet the parents firmly made up their minds that separation surgery was the best option. Nicole and Mark told CNN that they miss “simple everyday moments of parenting” like holding the twins and dressing them in regular clothes.

"These boys, they're raring to go," Christian says. "They can't stay like this any longer."