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 antiretroviral prophylaxis in Thailand (maternal antenatal lopinavir-based cART; nonbreastfed infants 4 weeks' postnatal zidovudine) were offered "antiretroviral intensification" (labor single-dose nevirapine plus infant zidovudinelamivudine-nevirapine for 2 weeks followed by zidovudinelamivudine for 2 weeks) if their antenatal cART was initiated ≤8 weeks before delivery. A negative birth HIV-DNA polymerase chain reaction (PCR) followed by a confirmed positive PCR defined intrapartum transmission. Before study initiation, we modeled intrapartum transmission probabilities using data from 3738 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) with/without intensification were estimated. After including the transmission data observed 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% (95% credibility interval 0.5-6.1) without intensification versus 0.3% (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. [ABSTRACT FROM AUTHOR]