Despite a great deal of feminist work that has highlighted its social construction, menstruation seems a self-evidently natural bodily process. Yet, how menstruation is defined or what counts as menstruation is rarely questioned. Examining menstruation alongside technologies that alter it highlights these definitional questions. In this article, I examine menstrual suppression through an analysis of medical journal articles and FDA advisory committee transcripts, paired with websites used to market menstrual suppression to consumers. Across these contexts (clinical research, FDA regulation, and advertising), new definitions of menstruation converged on a distinction between bleeding that occurs when women are taking hormonal birth control and when they are not. The case of menstrual suppression birth control pills provides an opportunity to study the work of redefining a biological process understood as quintessentially natural and deeply significant for gendered embodiment, as well as a challenge to consider both the social and material construction of gendered bodies.