What is zidovudine (Retrovir) used to prevent in HIV-positive pregnant women and their newborn babies?

Zidovudine (Retrovir) is used in HIV-positive pregnant women over 14 weeks of gestation to prevent maternal–foetal HIV transmission and for primary prevention of HIV infection in newborns; intravenous Retrovir is reserved for when oral treatment is impossible, except during labour and delivery.