Meet a Teenage Primordial Dwarf and Her Twin Sister Sienna "Sinny" Bernal and Sierra Bernal are the only known identical twins with a primordial dwarf in the pair. When Sinny was born, she weighed just over a pound and was nearly six weeks behind her sister in development. Doctors gave her a 10 percent chance of surviving. Courtesy of the Bernal Family

Meet a Teenage Primordial Dwarf and Her Twin Sister While her twin sister, Sierra, pictured here, went home after a few days in the hospital, Sinny spent the first 108 days of her life in intensive care. Primordial dwarfism is a form of dwarfism that results in a smaller body size in all stages of life beginning from before birth. Courtesy of the Bernal Family

Meet a Teenage Primordial Dwarf and Her Twin Sister Sinny was so small her mom could fit a wedding band around her wrist. "Her lungs were not fully developed, and she couldn't eat food yet, because everything wasn't developed," her mother Chrissy Bernal told "20/20." "Her skin was also really, really thin, so we couldn't touch her." Courtesy of the Bernal Family

Meet a Teenage Primordial Dwarf and Her Twin Sister The twins, seen here at 4-months-old, were born six weeks early. The family had to keep Sinny attached to an oxygen tank. Courtesy of the Bernal Family

Meet a Teenage Primordial Dwarf and Her Twin Sister Doctors initially identified Sinny as having a brain defect called Dandy-Walker syndrome before she was born. "When she was younger, actually, they didn’t know she was going to stop growing, ‘cause they didn’t really know she was a dwarf until she was older," Sierra told "20/20." The twins are pictured here at age 2. Courtesy of the Bernal Family

Meet a Teenage Primordial Dwarf and Her Twin Sister Sinny and Sierra, seen here at age 4, grew older, but Sinny wasn't growing in size. Remarkably, the true cause of Sinny's size deficit would remain undiagnosed for years. Courtesy of the Bernal Family

Meet a Teenage Primordial Dwarf and Her Twin Sister It was not until the twins were 6, that doctors first identified Sinny as a primordial dwarf. Courtesy of the Bernal Family

Meet a Teenage Primordial Dwarf and Her Twin Sister As the twins, pictured here at age 7, got older, their mother Chrissy Bernal said it became more difficult for her to find matching outfits. Courtesy of the Bernal Family

Meet a Teenage Primordial Dwarf and Her Twin Sister Unfortunately, Sinny is not without health problems. The bones in her ears are so small that she requires hearing aids. Sinny and her sister Sierra are seen here at age 9. Courtesy of the Bernal Family

Meet the Only Primordial Dwarf in the World With a Twin Sister Sinny and Sierra, pictured here with their mom, also dealt with things normal for girls their age, such as driving lessons, dreaming of a singing career, and boyfriends. "She's had one before I do," Sierra said of Sinny having a boyfriend. "I had three," Sinny responded. "I broke up with all three of them." Courtesy Bernal Family

Meet the Only Primordial Dwarf in the World With a Twin Sister Sinny's vision and hearing became worse as she reached her teens. She's also developing scoliosis and battling some learning disabilities. "I'm in a lot of pain, but I'm worth it, because I'm very tough, you know," Sinny told "20/20." Courtesy Bernal Family