Objective-Results regarding potential adverse effects of antiretroviral drugs during pregnancy are discrepant and few studies, most from Europe, have provided information about pregnancy outcomes of those already on treatment at conception. The aim of this study was to investigate the impact of antiretrovirals on pregnancy outcome according to the timing of treatment initiation in relation to pregnancy in a cohort of Brazilian HIV infected pregnant women.Methods-A prospective cohort of 696 pregnancies followed-up in one single center between 1996 and 2006 was studied. Patients in receipt of antiretrovirals before pregnancy were compared with those treated after the first trimester. The outcomes evaluated were preterm delivery (PTD): < 37 weeks; severe PTD (< 34 weeks); low birth weight (LBW): < 2500 g; very LBW: < 1500 g. Results-Patients on pre-conception use of ARV had higher rates of LBW (33.3% vs. 16.5%; p = 0.0002), and a similar trend for PTD (26.3% % vs. 17.7%; p = 0.09). Stratification by type of therapy (dual vs. HAART) according to timing of initiation of ARV showed that patients in use of preconception HAART have a higher rate of PTD (20.2% vs. 10.2%, p = 0.03) and LBW (24.2% vs. 10.2%, p = 0.002). After adjusting for several factors, pre-conception HAART was associated with an increased risk for PTD (AOR: 5.0; 95% CI: 1.5 -17.0, p = 0.009) and LBW (OR: 3.6; 95% CI: 1.7 -7.7, p = 0.001).Conclusions-We identified an increased risk for LBW and PTD in patients in receipt of HAART prior to pregnancy.