App usability, engagement, and postpartum weight retention: secondary analysis of the INTER-ACT randomized controlled trial

BackgroundPostpartum weight retention (PPWR) is a key contributor to long-term obesity risk and an important determinant of maternal health. Mobile health (mHealth) interventions may support postpartum change to a healthier lifestyle, but their effectiveness may depends on usability. This study examined whether perceived usability of the INTER-ACT app was associated with weight loss at 6 months postpartum and whether this association was mediated by app use frequency, motivational power, and implementation of lifestyle recommendations. MethodsThis secondary analysis used data from the intervention arm of the INTER-ACT randomized controlled trial (RCT), which provided postpartum lifestyle coaching and a supportive app to women with excessive gestational weight gain (EGWG). Participants (n=138) completed the process evaluation survey at 6 months postpartum, which included the System Usability Scale (SUS), perceived motivational power, and perceived implementation of lifestyle recommendations. Weight loss was calculated between 6 weeks and 6 months postpartum. Pearson correlations and parallel mediation analyses tested associations between usability, engagement-related variables, and weight loss, adjusting for maternal age, parity, and pre-pregnancy BMI. ResultsThe INTER-ACT app demonstrated moderate usability (mean SUS=60.6). Usability was positively associated with app use frequency (B=.05, p<.001), perceived motivational power (B=.003, p<.001), and perceived implementation of lifestyle recommendations (B=.01, p=.036). However, none of these were associated with weight loss at 6 months postpartum. Usability showed no total, direct, or indirect effect on weight loss. Gestational weight gain (GWG) was the only significant predictor (B=0.24, p=0.003). Higher usability was also associated with more positive and fewer negative emotional responses (p<.05). ConclusionApp usability was associated with engagement and positive emotional experience, but did not translate into reduced PPWR. GWG remained the main determinant of postpartum weight outcomes. Future interventions should prioritize preventing EGWG and complement this with emotionally supportive postpartum mHealth tools as part of a multi-faceted strategy.