The use of combination antiretroviral therapy for the prevention of mother to child transmission of HIV infection has achieved vertical HIV transmission rates of <1%. The use of these drugs is not without risk to the mother and infant. Pregnant women with HIV-infection are at high risk of preterm birth (PTB <37 weeks), with 2-4-fold the risk of uninfected women. There is accumulating evidence that certain combinations are associated with higher rates of PTB that others or no antiretroviral treatment. Understanding the pathogenesis of PTB in this group of women will be essential to target preventative strategies in the face of increasing HIV prevalence and rapidly expanding mother-to-child-transmission prevention programmes.