The overall aim of this project is to evaluate the effectiveness and implementation of a digital intervention to support parents to create healthy lifestyle behaviours from the start of their infant's life, in order to prevent overweight and obesity. Using a randomized controlled trial with a hybrid type I implementation-effectiveness design we will: 1. evaluate the effectiveness of the 16-month Health4Life intervention on: 1. Children's' dietary intake, screen time, physical activity, and sleep 2. Children's body mass index (BMI) z-scores 3. Parental self-efficacy 2. Evaluate the cost-effectiveness of the Health4Life app. 3. Evaluate and explore the implementation of the Health4Life app within primary child healthcare with regards to its: 1. acceptability, appropriateness, and feasibility according to child healthcare nurses. 2. satisfaction and usage by parents.