There is evidence to suggest the decline of trusting relationships in modern healthcare systems. The primary aim of this study was to investigate the role of trust in medical transactions in Thailand, using obstetric care as a tracer service. The paper proposes an explanatory framework of trust for further investigation in other healthcare settings. The study site was a 1300-bed tertiary public hospital in Bangkok which it provides two forms of obstetric care: regular obstetric practice (RP) and private obstetric practice (PP). Forty pregnant women were selected and interviewed using a set of guiding questions. A thematic analysis of the interviews was undertaken to generate understanding and develop an explanatory framework. It was found that patients' trust in obstetric services was influenced by their perceptions of risk and uncertainty in pregnancy and childbirth, and that these perceptions were linked to their social class. Social class also influenced the accessibility and affordability of care to patients. Middle class pregnant women with relatively high-level concerns about risk and uncertainty preferred using PP service as a means to achieve interpersonal trust. These women thought that an informal payment would provide the basis for interpersonal trust between themselves and the chosen obstetricians. In practice, however, obstetricians involved in PP rarely acknowledged this reciprocal relationship and hardly expressed the additional courtesy expected by patients. As a result, PP service only created an expensive impersonal trust that was mistaken as interpersonal trust by patients. Negative outcomes from PP often caused disappointment that could eventually lead to medical litigation. The study suggests that there are some negative impacts of PP within the health system. Negative experiences among PP users may undermine trust not only in the specific doctor but also trust in health professionals and hospitals more generally. Steps need to be undertaken to protect and strengthen existing impersonal trust, which combine institutional trust based on good governance and service quality with trust in the professional standard of practice. The explanatory framework developed through this study provides a foundation for further studies of trust in different specialties and care settings.