Background. The majority of HIV-positive women in South Africa are of reproductive age, and pregnancies among women using antiretroviral therapy (ART) are common. However, there are mixed data regarding the impact of ART on pregnancy outcomes. Objective. To examine the impact of ART on pregnancy outcome according to the timing of initiation of treatment. Methods. A retrospective cohort study was conducted among women delivering at a tertiary hospital from 1 October 2008 to 31 March 2009. Results. A total of 245 mothers were receiving ART: 76 mothers (31%) started ART pre-conception and 169 mothers (69%) started ART after the first trimester. No significant differences were observed in the rates of preterm delivery and low birth weight (LBW) between the pre- and post-conception groups (21% v. 24% and 21% v. 25%, respectively). Conclusion. In this cohort of women receiving ART in pregnancy, timing of ART initiation did not have any adverse effect on the measured pregnancy outcomes such as preterm delivery and LBW.