The abortion debate in the United States is often described in terms of its intractability, where opponents are locked in a struggle defined by apparently binarized dualisms (e.g., the right to choice vs. the right to life). Since 2022, when the “Dobbs decision” eviscerated the constitutional right to abortion, political actors and activists on both sides are increasingly making use of expert knowledge about the reproductive body to assert moral certainty about abortion. Homing in on the dynamics of this expert knowledge-making as both an anthropologist and physician-in-training, this ethnography addresses collective (un-)certainties about the reproductive body and the technologies that mediate it (specifically abortion and contraception). I trace contestations over the reproductive body in terms of an epistemological terrain I call “the menstrosphere,” in which many modalities of expertise—from biomedicine, to naturopathy, herbalism, and more—coexist. I ground the concept of the menstrosphere in a living archive, consisting of social media posts, online trainings, protocols for optimizing reproductive health, podcasts, mainstream media, and more, that charts the public life of expertise about the reproductive body. Across diverse sites—in a gynecologic skills workshop that uses a papaya to simulate the uterus, in the pastel-tinged aesthetics of Instagram, and even in IUDs suspended in uteri (including my own)—I reveal how expert certainties about the reproductive body are both produced and overdetermined by the intersecting legacies of (Christian) cis-heteropatriarchy; eugenic anxieties about race and population control; and capitalist logics that position women’s fertility as essential for economic production. I show how these legacies weave their way through the menstrosphere to tame the political potency of the reproductive body. These legacies also inform ready-made positions in our abortion politics in ways that are predictable and unproductive, and that increasingly make reproductive lives unlivable. Situated at the nexus of critical medical anthropology, feminist and queer theory, and science and technology studies, this dissertation argues that biomedicine should harness its expertise about the reproductive body to a new politics of personhood in the service of reproductive justice.