According to Plato’s Timaeus, people’s reproductive organs had minds of their own and since the uterus was not anchored down, it moved all over the body when it became agitated. Aretaeus of Cappodacia took it further and explicitly compared the uterus to an “animal within an animal.” Texts like the Hippocratic Corpus, the most commonly cited text on this belief, doesn’t state things nearly so strongly, suggesting that most Hippocratics believed its movements were confined to the torso, possibly only the lower section. They also believed that this could cause the vagina to close off, preventing the woman from menstruating.
One reason for this was the related belief that the uterus had no particular place within the body. After all, the penis was anchored externally and the male body was obviously the default. Ancient physicians did, however, have the evidence of uterine prolapse,** having seen it in animals. They then theorized that if the uterus could move downwards, it could also go the opposite direction.
There were two main theories. One stated that the uterus sought moisture. So if a woman had not had intercourse or been pregnant in too long, the uterus dried out and moved towards moister organs. The other theory, most notably found in Hippocratic texts and the Ebers Papyrus was that the uterus was attracted to sweet smells and repelled by foul ones. Thus, treatments tended to involve having a woman sit over something sweet smelling and having her inhale something nasty-smelling through her nose, with the recommendation that she get some guy to get her pregnant soon.
Almost anything that was physically wrong with a woman, any pain she experienced, any mood or reaction deemed “unusual” could be blamed on her uterus being out of place. Not all ancient physicians agreed on this theory. Soranus and Galen both opposed it, arguing that the uterus remained stationary. Despite this, however, the idea of the wandering womb remained a popular one in the medical field until the early modern period. It has since fallen out of favor, but it is impossible not to see traces of it in our attitudes today.
*Not too different from today, if you think about it.
**That is, when the uterus slips down into the vagina.
Plato, Timaeus 90e-91d - Perseus
Hippocrates, Places in Human Anatomy 47 - Diotima
Aretaeus of Cappadocia, On the Causes and Symptoms of Acute Diseases 2.11 - Perseus
The Ebers Papyrus - University of Chicago
Hanson, Ann Ellis. "Continuity and Change: Three Case Studies in Hippocratic Gynecological Therapy and Theory." In Women's History and Ancient History, edited by Sarah B. Pomeroy, 73-110. Chapel Hill, NC: University of North Carolina Press, 1991.
Dean-Jones, Lesley. "The Cultural Construct of the Female Body in Classical Greek Science." In Women's History and Ancient History, edited by Sarah B. Pomeroy, 111-137. Chapel Hill, NC: University of North Carolina Press, 1991.