'Vulnerable' is a term often used for pregnant women in need of extended antenatal care, although the term is not well defined. This study focuses on healthcare professionals' interpretations and understanding of vulnerability in pregnancy, including their own role, to understand the practices of interprofessional and intersectoral collaboration in antenatal care for vulnerable pregnant women. Intrepretive Description informed the methodology of the study and the theoretical framework was inspired by Symbolic Interactionism. It was found that definitions of vulnerability in pregnancy are fluid, being based on the healthcare professional's individual assessment of the pregnant woman´s personal resources, personal characteristics and psychological factors, and that these definitions also depend on the healthcare professional's role in relation to the pregnant woman. The different interprofessional teams' identification of what constitutes vulnerability in a pregnant woman was complex and relied on different components. Interprofessional collaboration was influenced by the relationships between professionals and sectors involved in antenatal care for pregnant women and was thereby influenced by the settings and structures in place for interprofessional and intersectoral collaboration. Insight into the healthcare professionals' perspectives of vulnerability in pregnancy can help develop and improve the interprofessional and intersectoral collaboration in antenatal care of vulnerable pregnant women and their unborn babies.