Objective: To explore whether improvements in physical activity following the MobileMums intervention were mediated by changes in Social Cognitive Theory (SCT) constructs targeted in the intervention (bather self efficacy, goal setting skills, outcome expectancy, social support, and perceived environmental opportunity for exercise). This paper also examined if the mediating constructs differed between initial (baseline to 6 weeks) and overall (baseline to 13 weeks) changes in physical activity. Methods: Secondary analysis of data from a randomized controlled trial involving 88 postnatal women