Greatest height differential in living identical twins (female)

The greatest difference in height between monozygotic twins is 38 cm (1 ft 3 in), in the case of Sienna “Sinny” Bernal at 132 cm (4 ft 4 in) and her sister Sierra (both USA) at 170 cm (5 ft 7 in) when measured in Tomball, Texas, USA, in 2018, at 20 years old. The “discordant” sisters were born on 22 December 1998, and Sinny's diminuitive stature is the result of a form of primordial dwarfism so uncommon that it isn’t formally classified.

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.