The postnatal period is a time of increased susceptibility to HIV infection and superinfection for postpartum women, sexual partners, and infants. This study examined the effect of a prevention to mother-to-child transmission of HIV intervention compared to standard care, and factors associated with unprotected last sex and inconsistent condom use at 12-months postpartum. Participants were N = 1399 HIV-infected women 6-30 weeks pregnant (M = 18 weeks (SD = 5.75)) assessed during pregnancy and 12-months postpartum. Women were aged an average of 28 (SD = 5.82); 48% reported 10-11 years of education. Older age (adjusted odds ratio [AOR] = 1.00) and being employed (AOR = 1.51) were associated with greater odds of unprotected sex at 12-month postpartum. Disclosure of HIV status (AOR = 0.64) and greater male involvement during pregnancy (AOR = 0.92) were associated with lower odds of unprotected sex at 12-month postpartum. HIV negative or unknown partner status (AOR = 0.45) and greater depressive symptoms (AOR = 0.97) were associated with lower odds of inconsistent condom use at 12-months postpartum. The intervention was not associated with reduced sexual risk behavior. Future studies should address male involvement and consider measurement of culturally tailored male involvement constructs for men South Africa. Perinatal women may require support for sexual communication and gender equity to reduce sexual risk.