BACKGROUND: WHO recommends that HIV infected women receive antiretroviral therapy (ART) minimally during pregnancy and breastfeeding ("Option B"), or ideally throughout their lives regardless of clinical stage ("Option B+") (Coovadia et al., Lancet 379:221-228, 2012). Although these recommendations were based on clinical trials demonstrating the efficacy of ART during pregnancy and breastfeeding, the population-level effectiveness of Option B+ is unknown, as are retention on ART beyond the immediate post-partum period, and the relative impact and cost-effectiveness of Option B+ compared to Option A (Centers for Disease Control and Prevention, Morb Mortal Wkly Rep 62:148-151, 2013; Ahmed et al., Curr Opin HIV AIDS 8:473-488, 2013). To address these issues, we conducted an impact evaluation of Zimbabwe's prevention of mother to child transmission programme conducted between 2011 and 2018 using serial, community-based cross-sectional serosurveys, which spanned changes in WHO recommendations. Here we describe the rationale for the design and analysis. METHODS/DESIGN: Our method is to survey mother-infant pairs residing in the catchment areas of 157 health facilities randomly selected from 5 of 10 provinces in Zimbabwe. We collect questionnaires, blood samples from mothers and babies for HIV antibody and viral load testing, and verbal autopsies for deceased mothers/babies. Using this approach, we collected data from two previous time points: 2012 (pre-Option A standard of care), 2014 (post-Option A / pre-Option B+) and will collect a third round of data in 2017-18 (post Option B+ implementation) to monitor population-level trends in mother-to-child transmission of HIV (MTCT) and HIV-free infant survival. In addition, we will collect detailed information on facility level factors that may influence service delivery and costs. DISCUSSION: Although the efficacy of antiretroviral therapy (ART) during pregnancy and breastfeeding for prevention of mother-to-child transmission of HIV (PMTCT) has been well-documented in randomized trials, little evidence exists on the population-level impact and cost-effectiveness of Option B+ or the influence of the facility on implementation (Siegfried et al., Cochrane Libr 7:CD003510, 2017). This study will provide essential data on these gaps and will provide estimates on retention in care among Option B+ clients after the breastfeeding period. TRIAL REGISTRATION: NCT03388398 Retrospectively registered January 3, 2018.