Participatory methodologies have become imperative when developing health promotion programmes. However, the concrete adoption of co-creation and its implications for intervention development are less reported. This article aims to convey how fidelity and adaptation were balanced in a structured intervention design by co-creating intervention components with various stakeholders. The intervention was part of the Face-it programme, which was initiated to prevent diabetes and increase the quality of life in women with prior diabetes during pregnancy by supporting the entire family's health practices. We relied on participatory methods, e.g. workshops using design games, role play and family interviews, as well as ethnographic fieldwork. Stakeholders comprised women with prior gestational diabetes mellitus and their families as intervention receivers and healthcare professionals, e.g. obstetricians, midwives and health visitors as potential intervention deliverers to shape intervention content. We used Bammer's stakeholder participation spectrum in research to describe how different stakeholders were engaged and with what implications for the intervention components. This article shows how an iterative co-creation process was (i) achieved through diverse involvement practices across stakeholder groups; and (ii) upheld both premises of the structured design (fidelity) and flexibility (adaptation) in developing intervention content and delivery. When adopting co-creation as a strategy for intervention development, we recommend using various engagement practices according to the role of stakeholders in the intervention and available resources to create ownership and sustainable intervention content.