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When Sienna “Sinny” Bernal entered the world in December 1999, she weighed just over a pound and fit neatly into the palm of her mother’s hand. Doctors gave her a 10 percent chance of living, but little did they know this diminutive girl had big plans.

After 108 days in a neonatal intensive care unit — battling a recurring case of pneumonia and surviving heart surgery — and, later, six years of visits to endocrinologists, neurologists and, finally, geneticists, Sinny and her family learned that she was one of the roughly 200 people in the world who have primordial dwarfism — a rare genetic condition. At 12 years old, Sinny stands just under 4 feet tall and weighs 28 pounds. She will likely remain this size for the rest of her life.

See photos of Sinny and Sierra through the years.

But aside from the obvious characteristics that set Sinny apart from other “tween” girls growing up in suburban Houston, there is one thing — or rather, one person — who makes her unique: her twin sister, Sierra. Sierra is an average-size girl, which makes the twins the only known pair in the world in which one is average size and one is a primordial dwarf.

Watch the full story on “20/20″ Friday at 10 p.m. ET.

The twins were born three months premature, and Sinny lagged six weeks behind her sister in development.

“Her lungs were not fully developed, and she couldn’t eat food yet,” their mother, Chrissy Bernal, said. “Her skin was also really, really thin, so we couldn’t touch her. And if she did live, they thought she’d be a vegetable.”

Sienna survived her infancy, but her small size continued to confound experts. It wasn’t until she was 6 years old that doctors in Texas identified her as a dwarf.

“Primordial dwarfism is really an umbrella or class of disorders,” Dr. Michael Bober, a leading researcher on primordial dwarfism at the Nemours/Alfred I. duPont Hospital for Children in Delaware, told “20/20.” He said it is not a precise diagnosis.

“A primordial dwarf is an individual who, from the very beginning of their growth in utero, grows at a slower rate. And when they’re born, and after birth, they continue to grow at a slower rate.”

The research in this area, Bober explained, is relatively new. His research team, along with the Potentials Foundation, is currently collecting DNA for a new registry of individuals with various forms of primordial dwarfism.

Now 12, Sinny and Sierra are active and vibrant young women. Sinny’s family openly discusses her condition at home, and she maintains a healthy sense of humor about her size.

“Oh, she’ll walk in the house [and say], ‘Dwarf in the house!’ Bernal said. “It’s the way you have to be, otherwise you’ll be depressed.”

Bernal blogs about Sinny’s life on SinnySized, and has written books about her own experience raising a daughter with special needs.

Despite her enthusiastic personality, Sinny continues to encounter medical problems. The bones in her ears are so small that she must wear hearing aids. Her family is currently investigating surgery to reconstruct the bones and offer a more permanent solution.

Because Sinny’s condition is so unusual, her family says there is no blueprint for the problems she may face.

“She is developing, and that’s a whole new bear that we’re not sure how to deal with,” Bernal said, adding that the social component of Sinny’s development had been a challenge as well. “We wonder, you know, what’s going to happen with homecomings and proms.”

Article source: http://feeds.abcnews.com/click.phdo?i=c5aca953060e80e34708187caf60abbf