This article is a case study examination of the role of taboos in the Middle Ages. It analyzes the menstrual taboo during the 12th century. In the Medieval Western World taboos have no structural value. On the contrary scientific and religious discourses bring forth rationalization techniques that countervail taboos. Nonetheless, taboos reappear sporadically in different cultural settings. The menstrual taboo is a good observatory of these contradictory evolutions. Medieval learned medicine promotes menstruation as an essential mechanism to upkeep the physiological balance of women. Therefore, it should not be impeded. Theologians and canonists mostly speak of menstruation when they criticize Jewish Law. The menstrual taboo is rooted in the Leviticus' prohibition abolished by Christianity. Penitentials of the Early Middle Ages reinstate taboos but the opinion of Gregory the Great carries more weight. The pope ruled in a famous statement that menstruation is a physiological issue out of women's control. There is no ill will and therefore no sin. Others theologians and canonists rationalize taboo as prophylaxis --- Medical knowledge explained that intercourse during menstruation could lead to leprous offspring ---. Learned knowledge tend to rationalize taboos. Everyday religion is more ambivalent. Churching become a common ritual during the 12th century. Churching finds its place amongst the ceremonies highlighting a Christian life. After childbirth women were prevented to go to church until their postpartum bleeding ended. However, a close examination of ritual and literary descriptions of childbirth shows that women are not feared because they are bleeding but because they are thrown in liminality where they could lose their humanity and devour their offspring. The trespassing of the frontier between man and beast is feared in the Middle Ages and is much more important in folklore than the blood taboo.
