When Chrissy Bernal first became pregnant in 1999, it took her doctors six months to realize that she would be having twins (video below).
One of them was so small that earlier scans didn't even pick her up, according to Little Things. One month later, the Texas mom gave birth to premature babies at 34 weeks during a risky C-section. Daughter Sierra came out looking healthy and normal for a premature baby, but Chrissy noticed that Sienna "Sinny" weighed little more than a pound and had a giant head and translucent skin. Chrissy thought she looked a little bit like an alien.
Nobody had any idea what was different about Sienna, but doctors said she had a 10 percent chance to live through the next few days. What's more, she would be a "vegetable" even if she did make it.
Sienna had a hole in her heart, and chronic lung disease, according to a 2011 Fox News report. She was also hard of hearing and had Dandy Walker syndrome, which means that she was missing part of her brain.
Popular VideoThis young teenage singer was shocked when Keith Urban invited her on stage at his concert. A few moments later, he made her wildest dreams come true.
But it wasn't until Sienna was six years old that the family even had an idea of what was going on with her. After seeing tons of perplexed doctors over the years, they found a visiting geneticist who finally figured out that Sienna is a primordial dwarf.
Primordial dwarfism is a very rare condition -- according to ABC News, there are only a few dozen known cases -- but what is even more unusual is to have one twin with primordial dwarfism and one without.
In fact, as far as the Bernal twins know, they are the only two in the world, and they relish every moment of it.
Popular VideoThis young teenage singer was shocked when Keith Urban invited her on stage at his concert. A few moments later, he made her wildest dreams come true:
"Being Sienna's twin is pretty cool," Sierra, now 18, told Fox News in 2011. "It makes me even more special. Being Sienna's twin has made me a strong person. Plus, I can pretty easily see who my real friends are because Sienna and I are a packaged deal. If you don't like her because she's different, then you don't get me."
Sienna is now a special needs advocate who runs a website and a couple of blogs dedicated to her unique condition. She also has a cooking website that teaches the differently-abled to cook safely and independently.
"What makes them different is what makes them wonderful!" Sienna's website says.