Although the beneficial effects of antiretroviral (ARV) therapy for preventing mother-to-child transmission are indisputable, studies in developed and developing countries have reported conflicting findings on the association between ARV exposure and adverse birth outcomes. We conducted a prospective observational study at 10 human immunodeficiency virus (HIV) care and treatment centers in Dar es Salaam, Tanzania. Multivariate log-binomial regression was used to investigate the associations between ARV use and adverse birth outcomes among HIV-negative HIV-exposed infants. Our findings demonstrate an increased risk of adverse birth outcomes associated with the use of highly active antiretroviral therapy during pregnancy. Further studies are needed to investigate the underlying mechanisms and identify the safest ARV regimens for use during pregnancy.