Bentley was born with his brain outside his skull. He shouldn't have survived.

Sierra Yoder’s hospital bag was light. There were no nappies, no bottles, no toys or blankets. Just a single onesie and a pair of socks – the only outfit her newborn son would ever have the chance to wear.

Or so she’d been told.

As reported by The Washington Post, a routine 22-week ultrasound revealed that Bentley suffered from a rare birth defect called encephalocele, which caused part of his brain to grow outside his skull. In most cases, the condition can cause mental and physical problems, compromised vision, seizures and muscle weakness in the limbs. In the worst, it’s fatal.

Doctors said Bentley would be among the latter. They told Sienna that even if he did survive, it would not be a life worth living.

But thanks to ambitious surgery, this is the little Ohio boy seven months on:

Image: Facebook.

At the time of diagnosis, the outlook for Bentley was so bleak that Sienna and her husband Dustin were encouraged to abort the pregnancy.

"That was the darkest part of the entire thing," she told The Washington Post. "We decided to abort that day. We didn't want him to come into the world and suffer."

A date was set, but then Sierra became attached to the thought of delivering her son, of cuddling him and showering him with love until the moment he would die.

"The night before the procedure, I told Dustin I couldn't do it," Yoder said. "He had a big sigh of relief. He was very happy."

And Bentley was born. He was never cleaned, never weighed; just passed around between family members for those first precious cuddles, cuddles that they also believed would be the last.

"We were all just staring at him. For the first four or five hours of his life, we were all just waiting for something to happen," Sierra said.

But after 36 hours of being held (Bentley hadn't been put down once), he was still alive.

Still thriving five months later, surgeons at the Boston Children's Hospital developed a radical plan to place the protruding parts of Bentley's brain back inside his cranium. See Bentley was quite unique among encephalocele patients, as the portions that were outside his skull contained functions he was actually using, like vision, motor skills and problem solving.

Using advanced 3D printers, the doctors developed a model of Bentley's skull on which to practise, before performing the surgery on May 24.

Just five hours later, he was in recovery.

The outlook for Bentley remains unclear, but doctors are optimistic, as are his parents.

"He is different, but we're going try to make it as un-different as possible, we're going to try to make sure that he has a normal life," she said in the above video. "I think he's going to put the rest of us to shame."