A pair of twins conjoined at the head have been successfully separated after a 20-hour operation.

Anias and Jadon McDonald, 13 months old, are recovering in the Bronx's Montefiore Hospital after their life-threatening operation on Thursday.

According to their surgeon - Dr James Goodrich - it is incredible they made it this far.

Half way through the operation, the veteran neurosurgeon considered stopping the procedure altogether when they discovered the boys shared far more brain tissue than they had expected - even despite using 3D imaging to prepare.

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With every cut, Anias' heart rate and blood pressure plummeted.

Eventually he found an opening and continued, successfully separating the two.

However, it means that while Jadon was wheeled out at 7am into the ward, Anias was still in surgery late on Friday morning.

Jadon and Anias recovering in their ward. Photo / Facebook
Jadon and Anias recovering in their ward. Photo / Facebook

He was finally returned to his family at 1pm on Friday. Doctors warned he could face severe physical issues.

Their parents Nicole and Christian, from near Chicago, Illinois, sat in the waiting room the whole time.

Nicole, 31, took to Facebook on Friday to explain all the details to the thousands of people that have followed the family's story.

'The overall atmosphere was one of celebration mixed with uncertainty,' she explained.