OBJECTIVE To compare time to achieve viral load <400 copies/ml and <1000 copies/ml in HIV-infected antiretroviral (ARV) -naive versus ARV-experienced pregnant women on highly active antiretroviral therapy (HAART). METHODS Retrospective cohort study. METHODS Three university medical centers, USA. METHODS HIV-infected pregnant women initiated or restarted on HAART during pregnancy. METHODS We calculated time to viral load <400 copies/ml and <1000 copies/ml in HIV-infected pregnant women on HAART who reported at least 50% adherence, stratifying based on previous ARV exposure history. METHODS Time to HIV viral load <400 copies/ml and <1000 copies/ml. RESULTS We evaluated 138 HIV-infected pregnant women, comprising 76 ARV-naive and 62 ARV-experienced. Ninety-three percent of ARV-naive women achieved a viral load < 400 copies/ml during pregnancy compared with 92% of ARV-experienced women (P = 0.82). The median number of days to achieve a viral load < 400 copies/ml in the ARV-naive cohort was 25.0 (range 3.5-133; interquartile range 16-34) days compared with 27.0 (range 8-162.5; interquartile range 18.5-54.3) days in the ARV-experienced cohort (P = 0.02). In a multiple predictor analysis, women with higher adherence (adjusted relative hazard [aRH] per 10% increase in adherence 1.29, 95% confidence interval [CI] 1.08-1.54, P = 0.01) and receiving a non-nucleotide reverse transcriptase inhibitor (NNRTI) -based regimen (aRH 2.48, 95% CI 1.33-4.63, P = 0.01) were more likely to achieve viral load <400 copies/ml earlier. Increased baseline HIV log10 viral load was associated with a later time of achieving viral load <400 copies/ml (aRH 0.60, 95% CI 0.39-0.92, P = 0.02). In a corresponding model of time to achieve viral load <1000 copies/ml, adherence (aRH per 10% increase in adherence 1.79, 95% CI 1.34-2.39, P < 0.001), receipt of NNRTI (aRH 2.95, 95% CI 1.23-7.06, P = 0.02), and CD4 cell count (aRH per 50 count increase in CD4 1.12, 95% CI 1.03-1.22, P = 0.01) were associated with an earlier time to achieve viral load below this threshold. Increasing baseline HIV log10 viral load was associated with a longer time of achieving viral load <1000 copies/ml (aRH 0.54, 95% CI 0.34-0.86, P = 0.01). In multiple predictor models, previous ARV exposure was not significantly associated with time to achieve viral load below thresholds of <400 copies/ml and <1000 copies/ml. CONCLUSIONS Pregnant women with ≥50% adherence, whether ARV-naive or ARV-experienced, on average achieve a viral load <400 copies/ml within a median of 26 days and a viral load of <1000 copies/ml within a median of 14 days of HAART initiation. Increased adherence, receipt of NNRTI-based regimen and lower baseline HIV log10 viral load were all statistically significant predictors of earlier time to achieve viral load <400 copies/ml and <1000 copies/ml. Increased CD4 count was statistically significant as a predictor of earlier time to achieve viral load <1000 copies/ml.