The notion of ‘evidence’ circulates in two realms of current maternity care: biomedical ‘evidence-based’ obstetrics and efforts to reform conventional obstetric practices. I observed that in California’s childbearing culture, ‘evidence’ is a boundary object that allows diverse actors to engage in related conversations despite fundamentally different assumptions about what evidence is or does. Sometimes these actors form productive hybrids and other times they talk past one another. This article uses recent work from the history and philosophy of science to distinguish the biomedical use of evidence, which is based on controlled experiments to prove cause and effect, from reformists’ use of evidence, which foregrounds patient outcomes. Using Stengers’s classification of doctors, charlatans, and curers, I discuss the role of rationality and experience in producing authoritative knowledge. Reformists’ use of evidence, in effect, challenges medical power dynamics on what they perceive to be the terms of medical authority itself; in doing so, it has the potential to fundamentally alter who is the primary beneficiary of medical protocols. The challenge is continuing to use evidence in a way that doesn’t simply ossify a new set of norms, but becomes increasingly capacious, flexible, specific, and patient centered.