Prevention of mother to child transmission of HIV is now possible through evidence-based strategies that reduce the risk of transmission to less than 2%. However, the effectiveness of these strategies depends on the mothers living with HIV adhering to HIV treatment and being retained in care. Difficulties in adhering to prevention strategies has resulted in mother to child transmission (MTCT) rates that remain unacceptably high. In Kenya, MTCT rates are still estimated at around 14% with over 6,000 newly infected children in 2016. The majority of pregnant women in Kenya are engaged in HIV treatment with an estimated 80% accessing ART during their pregnancy. Importantly, Option B+, a plan that provides pregnant women access to ART for life, is also now recommended by the World Health Organization and is being implemented in many countries, including Kenya. Despite these gains, poor adherence to PMTCT strategies remains a persistent problem- especially during the postpartum period. Indeed, current evidence estimates from 50% to as much as 81% of women become disengaged in care by six months after giving birth. To achieve zero vertical HIV transmissions and optimize maternal and infant health there is urgent need to address suboptimal adherence to PMTCT strategies. We aim to address this gap through an innovative approach that include the followings objectives: 1) Identify the challenges women face in adhering to prevention strategies by exploring how contextual, interpersonal, clinic and individual level factors change during key moments of transition postpartum, and, 2) Develop a Transitions Theory-based PMTCT and ART adherence intervention for postpartum women living with HIV. The study will consist of in-depth interviews with pregnant women living with HIV who we will follow up with for 3 months postpartum. The data will be used along with key informant interviews and focus group discussions to develop a Transition Theory-based intervention to improve PMTCT adherence and retention in care and ultimately improve maternal and infant health.