Collaboration between birth care and Preventive Child Health Care (PCHC) in the Netherlands is so far insufficient. The aim of the Connecting Obstetric; Maternity; Pediatric and PCHC (COMPLETE) study is to: (1) better understand the collaboration between birth care and PCHC and its underlying mechanisms (including barriers and facilitators); (2) investigate whether a new multidisciplinary strategy that is developed as part of the project will result in improved collaboration. To realize the first aim, a mixed-method study composed of a (focus group) interview study, a multiple case study and a survey study will be conducted. To realize the second aim, the new strategy will be piloted in two regions in an iterative process to evaluate and refine it, following the Participatory Action Research (PAR) approach. A prospective study will be conducted to compare outcomes related to child health, patient reported outcomes and experiences and quality of care between three different cohorts (i.e., those that were recruited before, during and after the implementation of the strategy). With our study we wish to contribute to a better understanding of collaboration in care and develop knowledge on how the integration of birth care and PCHC is envisioned by stakeholders, as well as how it can be translated into practice.