Introduction: HIV prevention interventions targeting couples are efficacious, cost-effective and a key strategy for preventing HIV transmission. Awareness of the full spectrum of relationship types and underlying complexities, as well as available support mechanisms in a given context, are critical to the design of effective couple-based interventions. Objective: This paper is based on a sub-analysis of a qualitative research study investigating HIV disclosure dynamics amongst pregnant women living with HIV in Durban, South Africa. The sub-analysis explored the nature of participants' social and relationship contexts and consequences of these dynamics on women's feelings of trust towards partners and perceptions of partner commitment. Methods: Between June and August 2008, we conducted in-depth interviews with 62 pregnant women living with HIV and accessing Prevention of Mother-to-Child Transmission (PMTCT) services in Durban, South Africa. Transcripts were coded for emergent themes and categories using a grounded theoretical approach. Results: The median age of participants was 26 years (interquartile range: 22-29 years). Three major. themes with accompanying sub themes were identified: 1) relationship types (sub themes included unmarried status, minimal cohabitation with partners, presence of concurrent relationships), 2) relationship quality/functioning (sub themes included low trust and expectation of partner commitment, relationship turbulence, and lack of communication/ability to negotiate protective behaviours), and 3) factors underlying the relationship functioning (sub themes included dynamics of concurrent relationships coinciding with concurrent pregnancies, gender roles and unequal relationship power, intimate partner violence or threat thereof, and lack of social support). Conclusions: Our research findings indicate a lack of many of the dyadic relationship elements underlying couple-counselling frameworks for successful risk reduction coordination. Understanding sexual behaviour and the accompanying relationship dynamics within different types of partnerships is crucial for the optimal design of couple-based HIV prevention interventions. (C) 2016 Elsevier Ltd. All rights reserved.