Abstract
Child undernutrition remains a global challenge, intricately linked to systemic inequities in access to maternal and child health and nutrition (MCHN) services. While SMS-based behaviour change communication (BCC) offers a scalable strategy to address this challenge, its effectiveness is often undermined by low and inconsistent user engagement. This study extends the impact evaluation of the mNutrition programme in Tanzania-a nationwide SMS-based BCC programme aimed at improving maternal and child nutrition (MCN)-to investigate what drives sustained, long-term engagement-an important but underexplored gap. The objective is to explore the mechanisms driving sustained engagement among mothers in rural Tanzania and how these are triggered by specific contextual factors and inequities. A realist approach was used, combining realist interviews with 40 sustained engagers (conducted across three sites between February and April 2019) and quantitative endline survey data. Retroductive analysis iteratively tested initial programme theories (IPTs) and derived Context-Mechanism-Outcome (CMO) configurations explaining sustained engagement. Findings showed sustained engagement resulted from the interplay between programme elements-such as content, SMS delivery (regularity, convenience, privacy), message tone (non-judgmental, supportive), and perceptions of the sender-and inequity-laden contextual realities, including time poverty, social isolation, undervaluation of women, gendered power dynamics within households and healthcare settings. The study concludes that digital BCC programmes must be context-sensitive and equity-oriented to achieve sustained engagement. Designing messages and delivery systems that reflect users' lived experiences and address structural vulnerabilities can enhance engagement and support more equitable nutrition outcomes.