AbstractIntroductionThe reproductive years can increase women’s weight-related risk. Evidence for effective postpartum weight management interventions is lacking and engaging women at this time is challenging. Following a promising pilot evaluation of the Supporting MumS intervention, we assess if theory-based and bidirectional text messages to support diet and physical activity behaviour change for weight loss and weight loss maintenance, are effective and cost-effective for weight change in postpartum women with overweight or obesity, compared with an active control arm receiving text messages on child health and development.Methods and analysisTwo-arm, parallel group, assessor-blind randomised controlled trial with cost-effectiveness and process evaluations. Women (n=888) with body mass index ≥25 kg/m2 and within 24 months of giving birth, recruited via community and NHS pathways through five United Kingdom sites targeting areas of ethnic and socioeconomic diversity. Women are 1:1 randomised to the intervention or active control groups, each receiving automated text messages for 12 months. Data is collected at 0, 6, 12 and 24 months. The primary outcome is weight change at 12 months from baseline, compared between-groups. Secondary outcomes include weight change (24 months) and waist circumference (cm), proportional weight gain (>5kg), BMI (kg/m2), dietary intake, physical activity, infant feeding and mental health (12 and 24 months respectively). Economic evaluation examines health service usage and personal expenditure, health-related quality of life and capability wellbeing to assess cost-effectiveness over the trial and modelled lifetime. Cost-utility analysis examines cost per quality-adjusted life years gained over 24 months. Mixed method process evaluation explores participants’ experiences and contextual factors impacting outcomes and implementation. Stakeholder interviews examine scale-up and implementation.Ethics and disseminationEthical approval obtained before data collection (West of Scotland Research Ethics Service REC 4 22/WS/0003). Results will be published via a range of outputs and audiences.Trial Registration [2a]:ISRCTN16299220(prospectively registered).Strengths and LimitationsThis is the first trial to examine the effectiveness and cost-effectiveness of a behavioural intervention for supporting weight loss in postpartum women with overweight or obesity that is delivered solely by text messages.The trial tests a novel evidence and theory-based text message intervention with embedded behaviour change techniques that is fully automated and incorporates two-way messaging to encourage engagement and delivery of specific behaviour change techniques.Researchers conducting participant recruitment and outcome data collection are blind to group allocation. Any situations where blinding is not maintained are recorded and reported. Due to the nature of the intervention, it is not possible to blind participants to randomisation.The trial includes an active control comparator to minimise disappointment bias and attrition related to randomisation experienced in previous weight loss intervention trials.(1)Home visits by the research team are offered to participants for data collection to facilitate participation in research assessments, helping to overcome barriers to taking part and address issues in participant retention seen in other postpartum trials.Administrative informationThis protocol is written in line with the SPIRIT checklist,(2) with recommended content indicated by the numbers in square brackets after section titles.Title [1]: Effectiveness and cost effectiveness of a 12 month automated text message intervention for weight management in postpartum women with overweight or obesity: protocol for the Supporting MumS (SMS) multi-site, parallel-group, randomised controlled trial.Protocol version [3]: Version 3.0 dated 10thMay 2023.Trial status: Opened to recruitment in April 2022, with the first participant randomised in May 2022 and participant recruitment completed in May 2023. The trial is in follow-up until May 2025, with primary outcome data collection to be completed in May 2024.