Barcroft
Sienna and Sierra Bernal are identical twins - but Sienna is half the size of her twin sister Sierra.
The twins' mother Chrissy Bernal was six months into her pregnancy when she realised she was having twins - baby Sienna was so small that she had not been picked up on previous scans.
But it was not until six years after her birth that Sienna was diagnosed with primordial dwarfism, a disorder which begins in the womb and means she has a smaller perfectly proportioned body size than others her age
Primordial dwarfism is so rare that that there are only 200 people in the world known to have the condition. Sienna is the only primordial dwarf in the world to have an average sized twin.
The girls, who are 13, share the same coloured hair and physical traits, but tiny Sienna, or 'Sinny' as her family calls her, weighs half her sister's weight. Standing four feet tall to Sierra's 5ft 1in, Sienna weighs 2st 6lbs, compared to her sister's 5st 7lbs.
The blonde-haired, blue-eyed teens love shopping, wearing make-up and giggling about boys.
"Most of the time I don't really care I'm different - it actually makes me feel special," says Sienna.
Sierra looks out for me and we do loads together - shopping, swimming and singing. She can be bossy but she shows me how to flirt with boys, stands up for me if I'm teased and reaches things when they're too high.
The sisters were born three months premature with Sienna weighing slightly more than one lb.
Barcroft
Doctor's warned mum Chrissy, 34, that Sienna had just a 10 per cent chance of surviving.
But after 108 days in neonatal intensive care, she was fit enough to join her family at their home near Houston, Texas.
But it wasn't until she was six years old that Sienna was diagnosed with primordial dwarfism.
Chrissy, an insurance agent who lives with her daughters, their step dad Joey and his son Austin, 17, only found out she was expecting twins six months into her pregnancy.
Incredibly, scans had not picked up Sienna because she was so small:
Sienna was five weeks behind in development and they said she might not survive - but I refused to believe the worst.
"I had a C-section at 34 weeks. It was terrifying, the girls were tiny and Sienna looked like an alien. She had a big head and her skin was see-though. We could hold her in one hand, she wore nappies that were smaller than an iPod, and we bought her dolls' clothes, but the girls' bond was immediate."
Little Sienna was so small that a wedding ring could fit around her arm, but after surgery to correct a hole in the heart and overcoming chronic lung disease, she was allowed home weighing just 3lb.
Despite her tiny proportions, Sienna attends the same school as her twin, where the pair have lessons in Taekwondo, hip hop dancing and cheer leading. Sienna has the learning age of an eight year old.
Although she attends special needs classes, the twins go shopping together and last year they both entered a beauty pageant.
"Sienna doesn't let it affect her - when she comes home she'll often shout, 'Dwarf in the house!'" says Chrissy.
"Though it sometimes gets her down that she can't play volleyball like Sierra and the boys don't show as much interest in her."
"In a lot of ways Sienna is just me, but smaller," says Sierra. "Like any sister, she's annoying especially when she steals my clothes - even though my tops come down to her knees.
"But I'll always protect her. If she's teased at school I'll tell the bullies it's not cool."
Sienna needs surgery to correct a curvature of the spine and the bones in her ears are so small she has to wear hearing aids.
"As Sierra gets more independent, I worry about how Sienna will be affected - stuff like going shopping on her own just wouldn't be safe because she's so small and has a lower learning age," says Chrissy.
"But I try to treat the girls as equals. Sienna has a real passion for life and makes everyone laugh. I'm so proud of both my girls."
What fabulous girls!
For more information and to help raise money for children with primordial dwarfism, visit Walkingwithgiants.org.
More on Parentdish: Meet the world's smallest woman