The reasons women have menstrual periods have fascinated and baffled medics and philosophers in equal measure for millennia. This is unsurprising, given the curious nature of a monthly episode of blood loss that happens to healthy women. The connection between periods and fertility was well known, but it was not until the twentieth century, when scientists understood the role of the monthly process of ovulation, that the process was fully explained. That did not stop (normally male) medical thinkers from offering theories about why women naturally bled each month. In formulating and promoting these theories, they paid little attention to the effects menstruation had on women’s bodies and mood and sidelined the experiences of many women who have to cope with painful or heavy periods routinely.