The literature on childbirth organisations focuses on their critique of medical definitions of birth practices, their efforts to promote 'natural' or 'normal' birth, their espousal of choice rhetoric and their relationship to feminism. It says little, however, about the practices these organisations use to achieve their aims. Our study of the United Kingdom, Ireland, Portugal and France explores the centrality of knowledge-based activities to childbirth activism. Through these activities, we show, organisations (i) elicit the emergence of a concerned public through constituting evidence about women's experiences of childbirth and obstetric practices; (ii) produce evidence about obstetric practices from women's perspectives; (iii) articulate this vision of obstetric practices with a critical appraisal of scientific literature; and (iv) make visible international networks of actors who share similar concerns and conceptions. Drawing upon our empirical data we propose the notion of evidence-based activism in order to capture the specificity of birth organisations' modes of engagement and to describe what they bring about. Through evidence-based activism childbirth organisations get involved in policy making and become recognised as legitimate stakeholders; reframe the issues at stake; open debate with other stakeholders; and bring about changes in the health system. Knowledge-based activities also shape the missions and objectives of the organisations.