Optimal use of antiretroviral drugs by pregnant women living with human immunodeficiency virus (HIV) is crucial to treat maternal HIV infection and prevent perinatal transmission of the virus effectively. Our goal was to describe national trends of antiretroviral (ARV) use during pregnancy among HIV-infected women living in the U.S. and enrolled in Medicaid. We used the 2000-2007 Medicaid Analytic eXtract (MAX) files to identify our study cohort. ARV use was defined as the dispensing of at least one ARV drug prescription during pregnancy based on Medicaid pharmacy claims. The prevalence of HIV was calculated, and temporal trends of ARV use during pregnancy were compared to the U.S. perinatal treatment guidelines. Predictors of ARV use during pregnancy were assessed using logistic regression models. From 1,106,757 pregnancies (955,251 women), 3083 (2856 women, 0.28%) were identified as HIV positive. We found striking regional variations in the prevalence of HIV and ARV prescription dispensing among pregnant women. The states with the highest HIV prevalence were Washington DC (5.8%), Maryland (0.90%), and New York (0.89%); all other states had a prevalence below 0.5%. A substantial fraction of women did not have any ARV dispensing throughout pregnancy (637 of 3083 (21%) pregnancies), and women with limited health care utilization were the least likely to have ARV dispensings. This finding calls for further research to better characterize HIV-positive women who are enrolled in Medicaid prior to pregnancy and yet have no ARV prescriptions so that appropriate interventions can be implemented.