BackgroundThere is growing evidence for the value of technology-based programs to support fathers to make positive transitions across the perinatal period. However, past research has focused on program outcomes with little attention to the mechanisms of impact. Knowledge of why a program works increases potential for replication across contexts.MethodsParticipants were 40 Australian fathers enrolled in the SMS4dads text-based perinatal support program (Mean age 35.11 (5.87). From a starting point between 16 weeks gestation and 12 weeks postpartum, they were sent a maximum of 184 text messages. An inductive approach was used to analyse post-program semi-structured interviews. The aim was to identify mechanisms of impact aligned to previously identified program outcomes, which were that SMS4dads: 1) is helpful/useful; 2) lessens a sense of isolation; 3) promotes the father-infant relationship; and 4) supports the father-partner relationship.ResultsWe identified two types of mechanisms: four were structural within the program messages and five were psychological within the participant. The structural mechanisms included: syncing information to needs; normalisation; prompts to interact; and, the provision of a safety net. The psychological mechanisms were: increase in knowledge; feelings of confidence; ability to cope; role orientation; and, the feeling of being connected. These mechanisms interacted with each other to produce the pre-identified program outcomes.ConclusionsIf the current findings are generalisable then, future mobile health program design and evaluation would benefit from explicit consideration to how both program components and individual cognitive and behavioural processes combine to elicit targeted outcomes.