Diprosopus: Two-faced conjoined twins (incompletely separated identical twins). The twins have almost complete fusion of their bodies with one set of limbs. Part or all of the face is duplicated. The condition usually results in stillbirth.
The ancient two-faced Mexican figurines known as the "pretty ladies of Tlatilco" are now thought to represent diprosopus. It is believed these early Mesoamerican figurines may, in fact, be "the oldest scientifically medical images in world history." Sculpted over a period of 500 years beginning in 1200 BC, these little statues show diprosopus with two faces side by side. This degree of accuracy in documenting the anatomical and pathological features of a human head does not appear for another two millennia until the 16th-century studies of Hans Baldung Grien and Andreas Vesalius who did anatomical drawings based on dissected cadavers.
The term "diprosopus" comes from the Greek "di-," two + "prosopon," face = two-faced.