In Salt Lake City a medical drama has been under way today as doctors work to separate a pair of 4-year-old conjoined twins in the first surgery of its kind -- an operation that doctors said could take as long as 24 hours.
Watch more of Deborah Roberts report Friday on "20/20"
The surgery is especially delicate because of the organs the twins share and because of their age.
Kendra and Maliyah Herrin share a liver, kidney, large intestine, pelvis and a pair of legs.
Because they are older than other conjoined twins who have been separated, these sisters are also very aware of what the operation means.
Eleven doctors and an army of nurses and technicians have spent most of the day separating the girls in a surgery their parents are confident will go well.
"It's going to be like a rebirth," said Jake Herrin, the girls' father. "Just seeing them born again. … They'll be in different bodies."
Each child will get one leg and later a prosthetic one. But it is the kidney that makes the surgery so unusual.
Kendra will get the kidney and Maliyah will have to live on dialysis for months until she's strong enough for a kidney transplant, possibly from her mother.
"I'd donate both my kidneys if that meant she would be able to live a healthy normal life for the rest of her life," Erin Herrin said. "I'd do anything for my children. I think most mothers would."
The fact that they share a single kidney is why doctors have waited so long to separate the twins.
"Very young children and infants do not do well on dialysis. And so the doctors wanted them to be a little bit older and a little bit bigger so that she would be able to tolerate that," said Bonnie Midget at Primary Children's Medical Center, where the surgery is taking place.
And because the twins are older and able to express themselves, a psychologist has been working with the girls to prepare them for their new life apart.
Even if today's surgery goes well, both girls will face years of additional surgery and rehabilitation.