Mobile health (mHealth) interventions are a promising approach to promote mothers' and fathers' health in the perinatal period. This may be particularly true for psychosocially burdened families who are poorly reached by current preventive services. Studies are needed that examine how user‐centered and evidence‐based mHealth interventions look like for this target group. The objective of this paper is to describe the iterative development process of the I‐PREGNO app intervention that aims to prevent unhealthy weight gain and to promote mental health in psychosocially burdened families during the perinatal period. The systematic content development process was divided into four stages. User needs were assessed through focus group discussions with psychosocially burdened mothers and healthcare professionals (HPs; stage I). In stage II, a prototype of the app was developed and evaluated through usability tests and a walkthrough with the target group and HPs (stage III). Finally, the behavior change techniques implemented in the app were assessed using an existing taxonomy (stage IV). The focus group discussions revealed that HPs as well as end‐users would benefit from an intervention that addresses psychosocial aspects (i.e., emotion regulation, coping) and links these to health behaviors. The identified needs of the target group during the perinatal period were combined with existing evidence‐based content and translated into 12 app modules. Most of the behavior change techniques used in these thematic modules were assigned to the clusters self‐monitoring, knowledge building, and goal planning. The I‐PREGNO app development process was guided by an iterative and user‐centered approach involving the target audience and a multidisciplinary team of experts. The findings provide valuable implications for the design and development of evidence‐based self‐guided mHealth for hard‐to‐reach groups during the transition to parenthood. The efficacy of the I‐PREGNO intervention will be evaluated in randomized controlled trials in routine care.