Background: Weight management is a priority for maternity services due to pregnancy risks and long-term health implications for women and babies. Pregnancy interventions significantly improve maternal diet, physical activity, weight, and some pregnancy outcomes. There are complex barriers, underpinned by self-efficacy, to midwives’ implementation of UK guidelines. The GLOWING intervention uses social cognitive theory (SCT) to address evidence-based barriers to practice, with the aim of supporting midwives’ implementation of guidelines into routine practice.Methods: An external rehearsal pilot cluster randomised controlled trial in four NHS Trusts (clusters) in England, UK. Clusters were randomised to intervention arm where all eligible midwives received the intervention, or control arm which received no intervention. A random sample of 100 midwives were invited to complete questionnaires pre- and post-intervention. UK guideline recommendations were grouped into communication-related behaviours (weight and risk communication) and support/intervention-related behaviours (diet/nutrition, physical activity, weight management, referrals/signposting). Questionnaires were developed using SCT constructs (self-efficacy, outcome expectancies, intentions, behaviours) and 7-point Likert scale. Data were converted to a 0-100 scale, where higher scores were more positive. Descriptive statistics compared intervention and control arms, pre- and post-intervention.Results: Seventy-four midwives consented and 68 returned questionnaires. Pre-intervention, self-efficacy for support/intervention-related behaviours scored lowest. In the control arm, there was limited difference between the pre- and post-intervention scores. Post-intervention, the mean scores were consistently higher in the intervention than control arm, particularly for support/intervention self-efficacy (71.4, SD 17.1 and 58.4, SD 20.1). Mean self-efficacy was higher post-intervention than pre-intervention for all behaviour categories in the intervention arm: weight communication (76.3, SD 16.7 vs. 67.2, SD 21.1), risk communication (79.4, SD 16.4, vs. 68.6, SD 14.9), diet/nutrition/physical activity (76.4, SD 16.0 vs. 49.3, SD 16.5), weight management (72.1, SD 18.3 vs. 48.3, SD 19.8), referrals/signposting (63.3, SD 26.0 vs. 47.9, SD 17.3), and consistently higher than controls.Conclusions: This study supports the theoretical models used to develop GLOWING, particularly that low self-efficacy is core to implementation barriers. Results suggest that the intervention is successfully targeting self-efficacy, which could have a positive impact on guideline implementation. A future definitive trial is required to determine effectiveness and cost-effectiveness. Trial registration: ISRCTN46869894, retrospectively registered 25/05/2016, http://isrctn.com/ISRCTN46869894