My research will provide a detailed understanding of the changes to the physical activity of household members after the arrival of a new baby. Physical activity is complex, and its analysis necessitates precise objective measurement using advanced quantitative methods. Also, insight into the contexts in which physical activity behaviours occur will be obtained using rigorous qualitative methods where practice theories will provide a novel theoretical lens. This mixed-methods approach will provide insights that are genuinely innovative and highly interdisciplinary which, through partnerships with Public Health England, Sport England, and Bristol City Council, will help inform policy and practice.