The McDonald twins are having a speedy recovery after their separation surgery. Pictured: A photo of conjoined twins in Lima in 2015. Reuters

Four weeks after a grueling 27-hour surgery to separate conjoined twins who had lived their entire lives connected at the top of their heads, the parents of the 14-month-olds finally saw the bare head of one of their sons at the hospital in New York City.

“It’s the most amazing thing. I just can’t even believe it,” Nicole McDonald, who stood beside her son Jadon’s bed when bandages were removed for the first time, said according to a Tuesday report from CNN. “And look at his little hair. On top, it’s growing in!”

Jadon and his brother Anias, who is recovering at a slower pace, are on pace to have the fastest recovery for craniopagus surgery. The previous record was eight weeks.

Read: The Curious Life Of Conjoined Twins

“Historically, this will be the fastest” recovery, lead surgeon James Goodrich reportedly told the family recently. Goodrich, who considered stopping the surgery hours into the operation when he became concerned about the brains being intertwined, has performed the surgery seven times.

When twins are born conjoined they generally share important organs. That can include sharing of the heart, liver, intestine or rectum but can also include sharing a brain or sharing a spine or pelvis. The first case of a successful separation of conjoined twins occurred in 2004 when Jade and Erin Buckles, who shared a liver, were separated in Maryland.

Conjoined twin births — even generally speaking and including other kinds of conjoined cases — aren’t very common. They occur roughly once every 200,000 live births, according to the University of Maryland Medical School. Once born, their survival is not guaranteed: The rate of survival is between 5 percent and 25 percent. Between 40 and 60 percent of conjoined twins are born stillborn and around 35 percent survive for a single day.

The case of Jadon and Anias is even rarer than even most conjoined twin births. Roughly 70 percent of all conjoined twins born are female.