Sirenomelia is almost always fatal in newborns. But Shiloh defied doctors' expectations.
Shiloh's fused legs were the visible evidence of her condition, but they were not the reason it was so dangerous.
"She was missing a whole series of organs including her uterus and her bladder, and her large intestine," Dr. Matthew Hand, a kidney specialist at the Barbara Bush Children's Hospital at the Maine Medical Center, who treated Shiloh, told "20/20" in 2008. "She had no vaginal vault or rectum, and no way for urine to get out of her body. Most of these babies die because of poor renal tissue makeup in their body."
The cause of the disease is a still a mystery. Doctors believe that blood circulation doesn't develop normally and the womb, kidneys and other organs don't form as they should.
Having had her first kidney transplant at age two, and more than 150 surgeries and years of kidney dialysis in the eight years that followed, she spent her life under the care of doctors.
Shiloh was one of only three people in the world known to have survived this condition for any length of time. She finally died of pneumonia.
"20/20" profiled Shiloh and her family in January 2008. Shiloh's parents, Leslie and Elmer Pepin, said they knew it was possible that their child would be affected by the syndrome before Leslie gave birth.
"So if she had it, we were worried we wouldn't be able to love her," said Leslie. "[We] went through all these emotions, like you couldn't even believe. But she came out, and she had the biggest, blackest eyes. I remember [Elmer] saying, "She's beautiful."
Doctors warned that the condition is almost always fatal for a newborn. When Shiloh did not die at home, as some had expected, her parents brought her to Dr. Hand.
"Matt was the first physician who looked at Shiloh and saw a little girl who needed help, instead of a medical case that was uncertain," said Elmer.
Hand deferred to the parents in settling on a course of treatment. "The first thing I said to them was, 'What is it you want out of this?' They said three things. The first was that they wanted her to be home as much as she could, to be surrounded by her family. The second was for her to live as long as she could. And the third is that when it came time for her to pass away that she would be surrounded by people who loved her, and that she wouldn't be alone. And I said, 'Okay, we'll work with that, and we'll do the best that we can.'"
Shiloh was a well-known personality in her home town of Kennebunkport, where she loved to go bowling with friends, including Samantha Mitchell and Zack Kellett, her cousins and frequent companions.
"All the bumps and nudges in her, in her path, she's just overcome them," said Kellett.
"All the kids love her," said Mitchell. "She's like their role model, basically."
"We've always understood that she's supposed to be here," said Leslie. "When nobody else has believed it, we believed it, because it's just the way she is."