One of the first reported cases of conjoined siblings was way back in 1100. Mary and Eliza Chulkhurst were quite wealthy and lived an impressive 34 years, considering their condition and medical care at the time [source: University of Maryland Medical Center]. But most of us are more familiar with conjoined twins who became sideshow spectacles, and a few of these cases are downright heartbreaking. Daisy and Violet Hilton, for instance, were continuously exploited as sideshow subjects [source: Pednaud]. But the physical development of conjoined twins still proves fascinating.
In identical twins, a single egg divides after fertilization. While most eggs that divide would go on to fully form two separate embryos, in conjoined twins the egg doesn't entirely split, and parts of both embryos — skin, organs and the like — are fused together. The condition only occurs in 1 of 200,000 live births; only 5-25 percent survive [source: University of Maryland Medical Center]. Not every conjoined pair is conjoined the same way. Thoracopagus twins share an upper torso and heart, and omphalopagus twins share a breastbone and waist and usually have a few shared organs. Conjoined twins that share a head (craniophagus twins) are the least common, at only 2 percent of conjoined pairs [source: University of Maryland Medical Center].