Introduction <p>The aim of this paper was to report on fidelity in the preconception phase of the Healthy Life Trajectories Initiative <i>Bukhali</i> randomised controlled trial with young women in Soweto, South Africa, covering fidelity of content, dose and delivery, as well as barriers to fidelity, and opportunities and solutions to overcome these barriers.</p> Methods <p>This mixed methods, descriptive study collected qualitative data from sessions with staff delivering the intervention component of the trial (referred to as ‘Health Helpers’) to report on content and delivery, and an open-ended survey about a key component of intervention delivery, Healthy Conversation Skills. Quantitative data on dose were collected from dosage data, structured observations (also providing data on delivery), and staff records.</p> Results <p>Health Helpers were generally positive about the relevance and relatability of the intervention materials, and highlighted the importance of simplifying, translating, contextualising and tailoring the content for participants. Adaptations to the dose and delivery of the intervention included shifting to individual sessions (versus groups), and increasing the number of telephonic compared to in-person sessions at the research site. Dosage data indicated a high level of fidelity in terms of sessions delivered for intervention and control arms, and regarding the delivery of multi-micronutrient supplements to intervention participants. Results from the structured observations also evidenced good fidelity, although challenges were noted with some aspects of the delivery of Healthy Conversation Skills.</p> Conclusions <p> These findings provide actionable strategies for designing, delivering, and scaling complex interventions that remain both rigorous and responsive to local realities.</p>