Doctors feared conjoined twins Kendra and Maliyah Herrin wouldn't survive after they were born sharing an abdomen, pelvis, liver, kidney and large intestine.
The sisters are now 17 and thriving, following an unprecedented 26-hour operation that left them each with one leg when they were aged four.
Now, a new BBC Three documentary shows Kendra – who kept their only kidney – and Maliyah going about their daily lives like any other teenager.
'When people first hear our story, they like to ask a lot of questions,' they said.
'But simply we feel like we're the same as everybody else, we just have a few things that are a little different.'
They were born conjoined in Salt Lake City, Utah. It took years of deliberating before their parents, Jake and Erin, agreed to the surgical separation.
Surgeons, at the time, had never separated twins with just one kidney. It required months of research and preparation.
Doctors warned it would give the girls independence and the chance of a longer life but carried a risk of death.
In the end, it went smoothly – they were out of the hospital within six weeks, though they had to endure arduous spine-straightening and plenty of medical visits for years.
The girls are thankful their parents chose to go ahead with the procedure. Maliyah told Barcroft TV: 'We are happy that our parents chose to separate us.'
Before the operation, the twins had to learn to get around together, each of them controlling one of the legs.
Kendra said: 'I just remember that I would always want to be in control so I would pretty much run over her and she would be on her head.'
The twins hadn't thought much about the potential risks of the surgery, even though their parents had explained what was happening.
Kendra said: 'We just called it "cut apart day", we didn't really know what it meant. We were afraid of every surgery though so when we went in for that surgery, I just remember crying.'
Kendra and Maliyah now take everything in their stride, adapting to strollers and crutches, and crawling so they can experience everything they want to.
Kendra and Maliyah were studying online for years, which was particularly important for Maliyah in the last couple of years as she underwent a second kidney transplant.
She got her first kidney from their mother at the age of five, and it lasted 10 years. But at 15, her body started to reject it, and she had to go on dialysis.
After a year-and-a-half on the waiting list, an anonymous donor came through, and Maliyah underwent surgery.
Now, almost a year on, Maliyah is in good health, and they have decided to go to public school to mingle with other kids their age.
One of their friends and classmates, Anabelle, told the BBC: 'They have taught me so much about going through trials and accepting them with grace and brave.'
In spite of their surgeries and health issues over the years, the sisters are at high school and living a typical teenage life.
Kendra said: 'High school has been really good so far.' Maliyah added: 'We've never been bullied at school. We're lucky.'
Although they share the same group of friends, the sisters have different personalities – Kendra is more outgoing.
They started their YouTube channel, Herrin Twins, three years ago and regularly vlog about their lives. Kendra said: 'We like making the videos just to make people positive.'