Today mother and infant are routinely separated directly after birth if there is a need of specialised treatment and care, despite of the significant and positive effects of skin-to-skin contact. Thus, there is a need of change in organizing the treatment and care in a way that minimizes separation. The aim is to evaluate the implementation and effect of a complex family-centred intervention based 107 on zero separation and couplet care. The intervention is rooted in the philosophy of family-centred care. Essentially, mother infant dyads will be admitted together, where they will receive couplet care by neonatal nurses. The study comprises a quasi-experimental trial and a qualitative process evaluation including a field study and two interview studies. Finally, a health economic evaluation will be conducted to assess the cost-effectiveness of this complex intervention. The intervention will take place at the Neonatal Intensive Care Unit at Hvidovre Hospital. The nurses will as a part of the intervention be educated to take care of both mother and infant and carry out the intervention. Five families with experiences from the Neonatal Intensive Care Unit and the Maternity Unit participates as patient and public representative in the project, as their experiences and ideas will provide an added value to the project. This study contribute with a new perspective on how to organize the treatment and care of a newborn family in a Neonatal Intensive Care Unit. The study will be the first to examine zero separation and couplet care within sick mother-infant dyads. The study will provide knowledge about how an intervention consisting of zero separation and couplet care can be feasible and acceptable, and what kind of effect and impact it will provide. It is expected that the study as a whole may impact and profile clinical nursing, as well as benefitting public health.