Courtesy of the double Boing, I learned that the NLM currently has an on-line exhibit featuring the history of conjoined twins called "From 'Monsters' to Modern Medical Miracles". The quotes in the title are telling.
If you have read Clark's The Nature of Monsters, you have been exposed to the ridiculous schools of thought that existed hundreds of years ago surrounding persons born outside of normal human development. A common belief was that outside influences - anything from seeing a stray cat to eating a particular food - had direct consequences on the growing fetus, and depending upon the extent of the mother's shenanigans, a good and righteous woman would give birth to a normal baby. And if she didn't, well...she must have done something untoward, somewhere.
"From 'Monsters' to Modern Medical Miracles" touches briefly on this in their history on conjoined twins, along with some interesting images and facts culminating in modern-day ideas on conjoined twins. The picture above shows Millie and Christine McCoy, two sisters who were also known as "The Two-Headed Nightingale". They wrote an autobiography called "History and Medical Description of the Two-Headed Girl", which I would simply love to have a read through someday, as I find their tale both tragic and beautiful.
The on-line exhibit can be found on the National Library of Medicine's Web site.