Although there were probably earlier incidents of births of conjoined twins, the earliest recorded reference was a set of identical conjoined boys (the Armenian Twins) in 945 in Constantinople, connected from the waist to the abdomen. Initially admired as a curiousity, they were later thought to be bad omens and exiled. An attempted surgical separation caused the death of one of the twins, with the survivor dying three days later. (reported in Bulletin on the History of Medicine, 58 in 1984).

Perhaps the most famous conjoined twins born before Eng and Chang were the Biddenden Maids, Mary and Eliza Chulkhurst of Biddenden, Kent, England, who were born to a wealthy family in 1100 and lived until 1134. They were joined at the hips (although illustrations sometimes mistakingly show they were also joined at the shoulders) and bequeathed their family fortune to the Church. For centuries following their death (until the early 1900's), an annual celebration was held in Biddenden in which biscuits displaying their images were distributed to the poor on Easter. Today, the 20 acres that the sisters bequeathed to the Church contains cottages known as the Chulkhurst Estate. Following the death of one sister, the second twin refused to be separated, supposedly saying, "As we came together, we will also go together," dying six hours later.

A set of conjoined boys joined at the back was born in Glasgow, Scotland in the late 15th Century (we've seen it reported as both 1475 and 1490) and survived for 28 years (either 1503 or 1518), dying five days apart. They lived under the patronage of the King of Scotland, James IV. The most complete information we've read on these Scottish Brothers was in the book, The Two-Headed Boy & Other Medical Marvels by Jan Bonderon (Cornell University Press, 2000). According to that author, the brothers lived from 1490-1518 and spoke eight different languages, English, Irish, Latin, French, Italian, Spanish, Dutch and Danish. They were also avid singers. They were dicephalus tetrabrachius twins (two torsos, 4 arms, 2 heads, 1 pair of legs).

There are also reports of a pair of girls born in 1495 near Worms who were joined at the forehead. A 1983 Journal of the History of Medicine and Allied Sciences essay relays the supposed origin of the birth occuring when two women (one of whom was pregnant) were having a conversation and had a collision with their heads. The pregnant woman became frightened and passed the mark of her concussion onto her unborn twins.