Background. Little is known about rates of incident pregnancy among HIV-positive women initiating highly active antiretroviral therapy (HAART). Methods. We conducted a retrospective clinical cohort study among therapy-na？ve women ages 18–45 initiating HAART between 1 April 2004 and 30 September 2009 at an adult HAART clinic in Johannesburg, South Africa. We used Poisson regression to characterize rates and rate ratios of pregnancy. Results. We evaluated 5,996 women who experienced 727 pregnancies during 14,095 person-years at risk. The overall rate of pregnancy was 5.2 per 100 person-years (95% confidence limits [CL] 4.8, 5.5). By six years, cumulative incidence of first pregnancy was 22.9% (95% CL 20.6%, 25.4%); among women ages 18–25 at HAART initiation, cumulative incidence was 52.2% (95% CL 35.0%, 71.8%). The strongest predictor of incidence of pregnancy was age, with women 18–25 having 13.2 times the rate of pregnancy of women ages 40–45 in adjusted analysis. CD4 counts below 100 and worse adherence to HAART were associated with lower rates of incident pregnancy. Conclusions. Women experience high rates of incident pregnancy after HAART initiation. Understanding which women are most likely to experience pregnancy will help planning and future efforts to understand the implications of pregnancy for response to HAART. 1. Introduction Women of childbearing age bear the largest burden of HIV in sub-Saharan Africa [1, 2]. This is especially true in South Africa, the country with the largest population of HIV-positive individuals in the world [3]. In South Africa, HIV prevalence among young women is three times that among young men [4], and a stabilizing overall prevalence of HIV may actually mask very high HIV incidence rates among rural and urban women in some parts of the country [5]. The overlap between pregnancy and HIV is even more striking, with very high rates of both HIV and pregnancy incidence reported among young women in parts of South Africa [5] and HIV prevalence reaching 40% among pregnant women in some age groups [2–4]. A recent publication reporting on multiple sites across Africa noted high HIV incidence rates among HIV-positive women in many settings, as well as a 70% increased hazard of pregnancy after initiation of highly active antiretroviral therapy (HAART) [6]. With international support to attain universal access to HAART [7] and a growing interest in “treatment-as-prevention” [8], an increasing incidence of pregnancy among women receiving HAART seems inevitable. While there are some limited data from the pre-HAART era about how
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