The increasing availability of antiretroviral therapy (ART) and drug regimens to prevent mother-to-child transmission (PMTCT) has probably changed the context of childbearing for people living with HIV. Using data from 2009–2010 community-based surveys in Nigeria and Zambia, this study explores whether women's knowledge about ART and PMTCT influences the relationship between HIV status and fertility preferences and contraceptive behaviour. The findings show that women living with HIV are more likely to want more children in Nigeria and to want to limit childbearing in Zambia compared with HIV-negative women. While there is no significant difference in contraceptive use by women's HIV status in the two countries, women who did not know their HIV status are less likely to use contraceptives relative to women who are HIV-negative. Knowledge about ART reduces the childbearing desires of HIV-positive women in Nigeria and knowledge about PMTCT increases desire for more children among HIV-positive women in Zambia, as well as contraceptive use among women who do not know their HIV status. The findings indicate that knowledge about HIV prevention and treatment services changes how living with HIV affects childbearing desires and, at least in Zambia, pregnancy prevention, and highlight the importance of access to accurate knowledge about ART and PMTCT services to assist women and men to make informed childbearing decisions. Knowledge about ART and PMTCT should be promoted not only through HIV treatment and maternal and newborn care facilities but also through family planning centres and the mass media.