Mental illness among parents may lead to negative consequences for the unborn child and its future. Pregnant women who experience mental illness are not typically one of Sweden's social service's focus groups, although voluntary preventive social work is an exception. The aim of this qualitative interview study was to investigate social workers' experiences of working with this preventive approach toward this specific group of women. The objectives in the study were what obstructive and facilitating factors social workers encounter, how their work with clients enable a social worker-client relationship, and lastly what results this preventive social work may have. Obstructive and facilitating circumstances were found on both an organizational and an interpersonal level. Social workers' possibilities of using their wide room for action were shown to be of importance to create and build a relationship with the pregnant woman. It was also discovered that preventive social work may lead to a number of positive consequences for both the mother-to-be and the coming child. Attachment between the two and the woman's belief in herself as a competent parent may develop, and the abilty to reach out and ask for help if there is a need in the future was thought to increase. Based on this study, it is possible to distinguish a number of conclusions: the first one being preventive social work results in beneficial consequences for both woman and child. Secondly, social workers' wide framework allows them to build a relationship with the pregnant woman, and this relationship based practice is crucial for preventive social work to take place.