Sienna Bernal: World"s only primordial dwarf twin looks up to her sister

'My twin sister is a dwarf': But 5ft 1in Sierra is the one who has been looking up to brave four-foot Sienna for all their 13 yearsBaby Sienna was so small she was not spotted on scans until six months into her mother's pregnancyDue to a rare brain abnormality, Sienna also has the learning age of an eight-year-old

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 means she has had a smaller body size than others her age since before she was born.

Now 13-years-old, four-foot-tall Sienna can't help but look up to her sister, Sierra who measures 5ft1in. And she will look up to her twin more and more over the coming years – as Sierra is unlikely to grow any taller.

Close bond: Sienna looks up to her twin Sierra who is more than a foot taller than she is

Close bond: Sienna looks up to her twin Sierra who is more than a foot taller than she is

Close bond: Sienna looks up to her twin Sierra who is more than a foot taller than she is

The blonde-haired, blue-eyed teens love shopping, wearing make-up and giggling about boys. But due to a rare brain abnormality Sienna also has the learning age of an eight-year-old.

Sienna said: 'Most of the time I don't really care I'm different – it actually makes me feel special.

'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 prematurely, with Sienna weighing slightly more than a pound.

Doctors warned their mother, Chrissy, 34, that Sierra had just a ten per cent chance of surviving. But after 108 days in neonatal intensive care Sierra was fit enough to join her family at their home near Houston, Texas.

Surprise: Sienna, left, was only picked up on ultrasound scans six months into mother Chrissy's pregnancy because she was so small

Surprise: Sienna, left, was only picked up on ultrasound scans six months into mother Chrissy's pregnancy because she was so small

Growing up together: The blonde-haired, blue-eyed teens love shopping, wearing make-up and giggling about boys. But due to a rare brain abnormality Sienna also has the learning age of an eight year old

Growing up together: The blonde-haired, blue-eyed teens love shopping, wearing make-up and giggling about boys. But due to a rare brain abnormality Sienna also has the learning age of an eight year old

Growing up together: The blonde-haired, blue-eyed teens love shopping, wearing make-up and giggling about boys. But due to a rare brain abnormality Sienna also has the learning age of an eight year old

It was only when she was six years old that Sienna was diagnosed with primordial dwarfism, a rare form of dwarfism which affects around 200 people in the world. And Sienna is the only primordial dwarf to have an average sized twin.

'In a lot of ways Sienna is just me, but
smaller. Like any sister, she's annoying especially when she steals my
clothes – even though my tops come down to her knees'

Chrissy, who lives with her daughters, their step-dad Joey and his son Austin, 17, said: '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,'

On top of the world: Sienna has special needs classes, but attends the same school as her twin where she also has lessons in Taekwondo, hip hop dancing and cheerleading

On top of the world: Sienna has special needs classes, but attends the same school as her twin where she also has lessons in Taekwondo, hip hop dancing and cheerleading

One pound Sienna was so small that a wedding ring could fit around her arm. She immediately had surgery to correct a hole in the heart, and after overcoming chronic lung disease was allowed home.

Sienna still needs surgery to correct a curvature of the spine and the bones in her ears are so small she has to wear hearing aids.

She has special needs classes, but attends the same school as her twin where she also has lessons in Taekwondo, hip hop dancing and cheerleading. Last year the sisters both entered a beauty pageant, too.

Chrissy said: 'Sienna doesn't let it
affect her – when she comes home she'll often shout, “Dwarf in the
house!”. 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.'

Chrissy, pictured with her daughters and their step-dad Joey says: 'Sienna doesn't let it affect her - when she comes home she'll often shout,

Chrissy, pictured with her daughters and their step-dad Joey says: 'Sienna doesn't let it affect her – when she comes home she'll often shout, “Dwarf in the house!”'

Sierra said: 'In a lot of ways Sienna is just me, but smaller. 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.'

Chrissy, an insurance agent, added: '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.

'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.'