Among women with HIV infection, pregnancy is a time when maintenance of maternal health and reduction of vertical HIV transmission are primary concerns. Few studies have examined adherence to Antiretroviral Treatment (ART) during pregnancy and in the postpartum period when the demands of childcare may significantly interfere with women's self-care behaviors. This study examined ART use and adherence in HIV-infected pregnant and postpartum women participating in the Women and Infants Transmission Study (WITS-IV) in the US. Adherence was assessed through a self-report interview during the third trimester of pregnancy and six-month postpartum. Data were also collected on demographics, biomedical markers and health related symptoms. During the third trimester visit, 77% (309/399) of women completed the self-report adherence measure; 61% (188/309) reported complete adherence. Factors associated with non-adherence included advanced HIV disease status, higher HIV-RNA viral load, more health-related symptoms and alcohol and tobacco use. At six-month postpartum, 55% (220/399) completed the measure; 44% (97/220) of these women reported complete adherence. Factors associated with non-adherence during the postpartum period were ethnicity, more health-related symptoms and WITS clinical site. Results of multivariate analyses using Generalized Estimated Equation analyses across the two visits revealed that more health-related symptoms, higher HIV-RNA viral load, increased alcohol use and clinical site were independently associated with ART non-adherence. These analyses indicate that medication adherence is more likely during pregnancy than postpartum in HIV-infected women, perhaps provoked by motivation to reduce vertical transmission and/or intensive antepartum surveillance. Further investigation is warranted to clarify factors implicated in women's decision-making process regarding ART medication adherence.