Conjoined 13-month-old twins Jadon and Anias McDonald were successfully separated Friday morning, their mother Nicole McDonald announced via Facebook. The twins underwent a 16-hour surgery led by Dr. James Goodrich at Montefiore Medical Center’s Children’s Hospital in the Bronx, New York. Though the surgery went well, the boys are by no means in the clear. Both are now in surgery to reconstruct the areas of their skulls that were 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,” McDonald wrote Friday. Concerned with the risk at one point during the surgery, the doctor debated stopping altogether, said McDonald.

The babies fared differently throughout the procedure. Jadon maintained strong vital signs throughout, while Anias’ blood pressure and heart rate dropped significantly. Arias has since stabilized and was put on medication to ensure his vitals remain steady.

Anias may not be able to move one or both sides of his body at first, his mother explained. “We are standing on the brink of a vast unknown,” she said.

The surgery was the hardest Goodrich has ever performed, McDonald relayed. It was the doctor’s seventh separation surgery and only the 59th cranial separation performed in the world since 1952, CNN reported.

“They go back to a 1-month-old,” Goodrich told CNN. “They have to learn to sit. They have to learn to roll. They have to learn to walk. They basically go through a yearlong period of a second infancy.”

Cases of twins born conjoined are rare, occurring one in every 200,000 births, according to Medical Daily. More than 40 percent of them are stillborn. Babies joined at the head, like Jadon and Anias, are the rarest type of conjoined twins, with only two percent born alive.

Conjoined twins that cannot be separated have lived full, if curious, lives, reported Medical Daily.