Introduction: Infants born to women living with HIV initiating combination antiretroviral therapy (cART) late in pregnancy are at high-risk of intrapartum infection. Mother/infant perinatal antiretroviral intensification may substantially reduce this risk. Methods: In this single arm Bayesian trial, pregnant women with HIV receiving standard of care ARV prophylaxis in Thailand (maternal antenatal lopinavir-based cART; non-breastfed infants 4 weeks postnatal zidovudine) were offered ‘antiretroviral intensification’ (labor single-dose nevirapine plus infant zidovudine-lamivudine-nevirapine for two-weeks followed by zidovudine-lamivudine for two-weeks) if their antenatal cART was ≤8 weeks before delivery. A negative birth HIV-DNA PCR followed by a confirmed positive PCR defined intrapartum transmission. Prior to study initiation, we modeled intrapartum transmission probabilities using data from 3,738 mother/infant pairs enrolled in our previous trials in Thailand using a logistic model, with perinatal maternal/infant antiretroviral regimen and predicted viral load at delivery as main covariates. Using the characteristics of the women enrolled who received intensification, prior intrapartum transmission probabilities (credibility intervals; CrI) with/without intensification were estimated. After including the observed transmission data in the current study, the corresponding Bayesian posterior transmission probability was derived. Results: No intrapartum transmission of HIV was observed among the 88 mother/infant pairs receiving intensification. The estimated intrapartum transmission probability was 2·2% (0·5-6·1) without intensification versus 0·3% (95%CrI 0·0-1·6) with intensification. The probability of superiority of intensification over standard of care was 94·4%. Antiretroviral intensification appeared safe. Conclusion: Mother/infant antiretroviral intensification was effective in preventing intrapartum transmission of HIV in pregnant women receiving ≤8 weeks antepartum cART.