Background <p>Globally and in Sweden childhood overweight and obesity remain major public health issues. The first 1000 days (i.e., conception-24 months) has been identified as the period of greatest developmental plasticity, with the maximum capacity to influence health throughout life. As obesity-related behaviours, including excessive screen time and poor dietary habits, have been found to establish in infancy and continue into early childhood, this demonstrates the need for early life health promotion interventions. Therefore, the aim of this study is to evaluate the effectiveness of and explore the implementation of a digital intervention (Health4Life app) designed to support parents in creating healthy diet and movement behaviours from the start of their infant’s life, to promote health and prevent overweight and obesity.</p> Methods <p>A two-arm parallel open-label randomized controlled trial, using a hybrid type I implementation-effectiveness design will be conducted. Families will be recruited at the 6-week routine visit at participating child health service (CHS) centers (<i>n</i> = 504). If parents’ consent to participate families will be randomized in a 1:1 ratio into the control or intervention group. Both groups receive standard care; however, the intervention group also receives access to the 16-month Health4Life app-based intervention. Primary outcomes are children’s dietary intake (questionnaire), physical activity and sleep (accelerometry), and screen time (questionnaire). Secondary outcomes are children’s body mass index z-scores (measured at CHS) and parental self-efficacy for promoting health behaviours (questionnaire). The implementation outcomes being explored are acceptability, appropriateness, and feasibility according to CHS nurses (semi-structured interviews) as well as parental satisfaction and usage of the Health4Life app (questionnaires and semi-structured interviews).</p> Discussion <p>An important mission of CHS is to support parents in establishing healthy diet and movement behaviours, and the Health4Life app is designed to support this by providing early and tailored guidance to families. If shown to be effective as well as acceptable, appropriate and feasible, Health4Life has the potential to be implemented as a complement to CHS nurses’ routine work to promote children’s health and development in accordance with the Swedish national child health monitoring program.</p> Trial registration <p>This trial was registered at Clinicaltrials.gov (ID NCT06798597) on January 29th, 2025.</p>