Maternal antiretroviral therapy (ART) is a critical intervention in the prevention-of-mother-to child transmission (PMTCT) of HIV. In South Africa, many HIV-infected pregnant women commence ART late in pregnancy, and as a result, the duration of ART prior to delivery is often insufficient to prevent vertical transmission. To address this, we designed an intervention for the rapid initiation of ART in pregnancy (RAP), where patient's ART preparation occurred during rather than before treatment commencement. Here we report on the acceptability and the challenges of the RAP programme. We conducted 7 key informant and 27 semi-structured interviews with RAP participants. Participants were purposefully selected based on ART-eligibility and stage in the pregnancy to post-partum continuum. Interviews were conducted in participants' home language by trained fieldworkers, with key informant interviews conducted by the study investigators. The data were analysed using a framework analysis approach. Rapid initiation in pregnancy was acceptable to the majority of programme participants and protection of the woman's unborn child was the primary motivation for starting treatment. The key barrier was the limited time to accept the dual challenges of being diagnosed HIV-positive and eligible for life-long ART. Truncated time also limited the opportunity for disclosure to others. Despite these and other barriers, most women found the benefits of rapid ART commencement outweighed the challenges, with 91% of women initiated onto ART starting the same day treatment eligibility was determined. Many participants and key informants identified the importance of counseling and the need to make an informed, independent choice on the timing of ART initiation, based on individual circumstances. Acceptance of ART-eligibility improved with time on the programme, however, as women's principal reason for initiating ART was protection of the unborn child, monitoring and supporting adherence during the post-partum period will be critical.