Background: Suboptimal retention may undermine the individual and public health benefits of immediate lifelong antiretroviral therapy (ART) for HIV-positive pregnant women (i.e., Option B+). We evaluated the effect of an intervention called the B-plus Enhanced Adherence Package (BEAP) on early ART uptake and short-term retention among newly diagnosed HIV-positive pregnant women in Lusaka, Zambia.Methods: Pregnant women at 3 public sector facilities in Lusaka urban district who were 18+ years old, HIV-positive, and ART naïve were enrolled between March 02, 2017 and December 18, 2017 and randomized 1:1 to BEAP versus standard of care (SOC) services. All participants were eligible for Option B+ per national guidelines. BEAP involved early (within 1-2 weeks of eligibility for option B+) phone calls and home visits from a male-female pair of community health workers (CHW) to provide additional counseling, engage male partners in HIV testing, and follow-up any missed ART appointments. CHW documented all BEAP activities. The primary outcome, initiation and retention on ART per pharmacy records at 30 days after Option B+ eligibility, was compared by study arm in an intention to treat (ITT) analysis using logistic regression. In a per protocol analysis we also assessed the impact of BEAP on initiation and retention among those women who were confirmed to have received BEAP services. In pre-planned subgroup analyses, we also evaluated the BEAP's impact according to participant age, parity, baseline HIV partner disclosure status and baseline CD4+ count.Findings: Among 454 women enrolled, 229 were randomized to BEAP and 225 to SOC. The two arms were balanced with respect to age (p=0.16) and gestational age (p=0.09). Within 30 days of Option B+ eligibility, 445 (98.2%) initiated ART; however, only 369 (82.9%) of those women who started ART were still in care at 30 days. In ITT analysis, the proportion initiating and remaining on ART at 30 days was similar between women randomized to BEAP versus SOC (82.5% versus 80.4%; crude relative risk [RR], 1.03; 95% confidence interval [CI], 0.91-1.16). After exclusion of 92 participants (40.2%) due to not receiving BEAP services based on CHW documentation and 1 participant due to ineligibility, BEAP was associated with being on ART at 30 days versus SOC (91.9.0% versus 80.4%; crude RR 1.14; 95% CI, 1.02-1.29). No significant variation was found in the effect of BEAP in subgroup analyses based on parity, age, baseline HIV disclosure status, and/or baseline CD4+ count.Interpretation: Uptake of Option B+ within 30 days of eligibility was nearly universal among HIV-positive pregnant women in urban Zambia. Women who received the BEAP intervention were statistically more likely to be on ART at 30 days compared to those receiving standard services. This difference, though modest, supports the potential of targeted early interventions to optimize the implementation of Option B+.Trial Registration: The trial was also registered at ClinicalTrials.gov (trials number NCT02459678).Funding Statement: This research has been supported by the President's Emergency Plan for AIDS Relief (PEPFAR) through the United States Centers for Disease Control and Prevention (CDC) under the terms of cooperative agreement number U01GH000530. Additional investigator support was provided by the National Institutes of Health (K24 AI120796).Declaration of Interests: MM declares that the Centre for Infectious Disease Research in Zambia (CIDRZ), received a grant from the Centers for Disease Control and Prevention (CDC) (grant number: GH000530) to carry out this study. The rest of the authors declare no conflict of interest.Ethics Approval Statement: Approval for the study was obtained from the University of Zambia Biomedical Research Ethics Committee (Lusaka, Zambia) and the University of North Carolina at Chapel Hill Instituitional Review Board (Chapel Hill, NC, USA). The study was reviewed in accordance with the Centers for Disease Control and Prevention (CDC) human research protection procedures and was determined to be research, but CDC investigators did not interact with human subjects or have access to identifiable data or specimens for research purposes. Women who were eligible and agreed to participate provided written informed consent.