How women are cared for while pregnant and having a new baby can have profound and lasting effects on their health and well-being. While mainstream maternity care systems aspire to provide care that is woman-centred, women with fewest social and economic resources often have reduced access. Community-based doula support programs offer complementary care for these women and are known to, on average, have positive outcomes. Less understood is how, when and why these programs work. A realist evaluation of an Australian volunteer doula program provided for women experiencing socioeconomic adversity explored these questions. The program provides free non-medical, social, emotional, and practical support by trained doulas during pregnancy, birth and new parenting. This paper reports the testing and refinement of one program theory from the larger study. The theory, previously developed from key informant interviews and rapid realist review of literature, hypothesised that the cultural matching of woman (client) and doula led to best outcomes. This was tested in realist interviews with women and focus groups with doulas, in January–February 2020. Seven English speaking, and six Arabic speaking clients were interviewed. Two focus groups were conducted with a total of eight doulas from diverse cultural and professional backgrounds. Data were analysed in NVivo. The study found cultural matching to be valued by some but not all women, and only when the doula was also genuinely interested, kind, timely and reliable. These approaches (with or without cultural matching) generate trust between the doula and woman. Trust theory, reflexivity theory and social relations theory supported explanatory understanding of the causal contribution of a doula knowing what it takes to build trust, to a woman deciding to trust her doula.