BACKGROUND A high delivery maternal plasma HIV-1 RNA level (viral load [VL]) is a risk factor for mother-to-child transmission and poor maternal health. OBJECTIVE To identify factors associated with detectable VL at delivery despite initiation of highly active antiretroviral therapy (HAART) during pregnancy. METHODS Multicenter observational study. (ClinicalTrial.gov: NCT00028145). METHODS 67 U.S. AIDS clinical research sites. METHODS Pregnant women with HIV who initiated HAART during pregnancy. METHODS Descriptive summaries and associations among sociodemographic, HIV disease, and treatment characteristics; pregnancy-related risk factors; and detectable VL (>400 copies/mL) at delivery. RESULTS Between 2002 and 2011, 671 women met inclusion criteria and 13.1% had detectable VL at delivery. Factors associated with detectable VL included multiparity (16.4% vs. 8.0% nulliparity; P = 0.002), black ethnicity (17.6% vs. 6.6% Hispanic and 6.6% white; P < 0.001), 11th grade education or less (17.6% vs. 12.1% had a high school diploma; P = 0.013), initiation of HAART in the third trimester (23.9% vs. 12.3% and 8.6% in the second and trimesters, respectively; P = 0.003), having an HIV diagnosis before the current pregnancy (16.1% vs. 11.0% during the current pregnancy; P = 0.051), and having the first prenatal visit in the third trimester (33.3% vs. 14.3% and 10.5% in the second and third trimesters, respectively; P = 0.002). Women who had treatment interruptions or reported poor medication adherence were more likely to have detectable VL at delivery. CONCLUSIONS Data on many covariates were incomplete because women entered the study at varying times during pregnancy. CONCLUSIONS A total of 13.1% of women who initiated HAART during pregnancy had detectable VL at delivery. The timing of HAART initiation and prenatal care, along with medication adherence during pregnancy, were associated with detectable VL at delivery. Social factors, including ethnicity and education, may help identify women who could benefit from focused efforts to promote early HAART initiation and adherence. BACKGROUND U.S. Department of Health and Human Services.