Background: Inequality in health can have profound short- and long-term effects on a child’s life. Infants develop in a responsive environment, and the relationship between mother and infant begins to develop during pregnancy. The mother’s ability to bond with the fetus and newborn child may be challenged by mental health issues which can cause impaired functioning and poorer health outcomes. Families with complex problems need interdisciplinary interventions starting in early pregnancy to be prepared for motherhood and to ensure healthy child development. This study aims to examine the effects of an early and coordinated intervention (the Family Clinic and Municipality (FACAM) intervention) offered to vulnerable pregnant women during pregnancy and the child’s first year of life on the mother-child relationship, maternal social functioning, mental health, reflective functioning, well-being, parental stress, and the development and well-being of the child. Methods: The study is a prospective randomized controlled trial where we will randomize 320 pregnant women enrolled to receive antenatal care at the family clinic at Odense University Hospital, to either FACAM intervention or usual care. The FACAM intervention consists of extra support by a health nurse or family therapist during pregnancy and until the child starts school. The intervention is most intensive in the first 12 months and also includes attachment-based support provided either individually or in groups. The participants are assessed at baseline, and when the infant is 3 and 12 months old. The primary outcome is maternal sensitivity measured by the Coding Interactive Behavior (CIB) instrument. Secondary outcomes include prenatal parental reflective functioning, mental well-being, depressive symptoms, breastfeeding duration, maternal satisfaction, child development, parent competence, parental stress, and activities with the child. Discussion: The trial is expected to contribute knowledge about the effect of early coordinated support in antenatal and postnatal care for vulnerable pregnant women and their families. Trial registration: ClinicalTrials.gov NCT03659721. Registered on September 6, 2018