First twin with primordial dwarfism

The first – and to date only – documented case of identical twins in which one sibling has primordial dwarfism and the other is average height is the Bernal twins, Sienna ("Sinny") and Sierra (both USA, b. 22 December 1998), of Tomball, Texas, USA. At the age of 20 years old, Sinny stood 4 ft 4 in (132 cm) tall, while twin Sierra was 5 ft 7 in (170 cm).

The twins were born prematurely – with Sinny weighing just under 1 lb 8 oz (0.68 kg) and Sierra weighing 4 lb 4.8 oz (1.95 kg) – but it wasn't until she was six years old that Sinny was diagnosed with primordial dwarfism. This rare form of proportional dwarfism (affecting fewer than 200 people) is a class of disorders typically characterized by severe intrauterine growth retardation (IUGR); this means that restricted growth begins at the fetal stage and that infants are born smaller than average. Sinny's form of primordial dwarfism has yet to be determined, and may be unique to her. She was also born with Dandy-Walker Syndrome, a rare congenital disorder that affects brain development.

Sinny is an advocate for the Equal Restroom Access for Little People Movement, working alongside her mother to challenge public places to accommodate people of restricted growth. Sierra has embarked on a career as a country singer and is a member of the Country Music Association. With 1 ft 3 in (38 cm) between them, the sisters have the greatest known height differential in identical twins.