BackgroundIn healthcare, there is increasing recognition of the importance of developing and testing strategies to scale effective interventions. The NHS long-term plan (2019) acknowledges that often a gold standard approach to a problem already exists somewhere within the NHS, however, it has not been replicated widely across the system.MethodsWe describe the approach and process measures for national scaling of PROMPT (Practical Obstetric Multi-Professional Training) across 12 obstetric-led maternity units in Wales. PROMPT is an evidence-based training package for local maternity staff, previously associated with improvements in maternal and neonatal outcomes, reduction in litigation related to preventable harm and improved safety culture. PROMPT has previously been disseminated internationally using a train-the-trainer model. However, this has been associated with variations in uptake, fidelity and impact. In Wales, the project was supported by Welsh Government, and a structured scaling plan was developed, encompassing ongoing implementation support from a multi-professional team.ResultsPROMPT was successfully implemented in all obstetric led units in Wales, with 326 local PROMPT facilitators trained, and 82.5%–100% of maternity staff attended a local PROMPT course in the first 15 months of the project (January 2019–March 2020). All training courses included evidence-based authentic elements, and 93% of courses in the first year (100/107) were supported by a national implementation team, providing coaching, implementation support and quality assurance.ConclusionsAuthentically scaling up complex interventions is a significant challenge. To replicate the improved outcomes demonstrated by PROMPT, intervention reach and fidelity must first be demonstrated.In this national scaling project, our scaling methodology led to the successful implementation of PROMPT across all health boards in Wales. Additionally, we demonstrated reduced variation in adoption, reach, timescale and intervention fidelity between maternity units with varying readiness for change, which had been difficult in two previous large-scale PROMPT implementation projects.