A 20 hour, $2.5 million surgery performed by neurosurgeon Dr. James Goodrich successfully separated 13-month-old craniopagus twins Anias and Jadon McDonald on Thursday. The boys' case is extremely rare - twins conjoined at the cranium occur in only one in 2.5 million births.

"This is about as complicated as it gets," Goodrich told CNN.

The twins will likely suffer developmental issues as brain tissue had to be separated. 'It's a bit surreal to sit here and type this...I should feel so happy...TWO SEPARATE BABIES!!!...and yet I ache with the uncertainty of the future," Anias and Jadon's mother Nicole wrote on Facebook. "I didn't cry until the surgeon's left the room. I was barely able to even utter the words "thank you" because of the pit that still sits heavy in my stomach. We are standing on the brink of a vast unknown."

Anias and Jadon will recover in intensive care for at least 72 hours. They will also require months of rehab.