Background:  The majority of pregnant women who participate in prevention of mother-to-child transmission of HIV (PMTCT) programs are HIV-uninfected. Keeping these HIV-sero-negative women uninfected is an important component of efforts to eradicate pediatric HIV infections. However, there is increasing evidence that suggests relatively high HIV incidence (1.3 to 10.7 per 100 women-years) during pregnancy and the immediate postpartum period. Defining co-factors for HIV incidence during pregnancy and the postpartum period is a useful first step toward development of strategies to decrease HIV acquisition during this period.

Methods:  Mothers accompanying their infants for routine 6-week immunizations were tested for HIV at 6 maternal–child health clinics in Nairobi and Western Kenya. Prior to testing, mothers completed a questionnaire that included assessment of sociodemographic characteristics, obstetric history, HIV risk perception, and participation in perinatal HIV prevention programs during the last pregnancy. HIV-negative mothers who seroconverted were compared to those who did not.

Results:  Of 2135 mothers who had tested HIV-seronegative antenatally, 2035 (95.3%) accepted HIV re-testing. Of these, 53 (2.6%) were HIV-seropositive yielding an estimated HIV incidence of 6.8 (95%CI 5.1 to 8.8) per 100 woman-years. Mothers who seroconverted were more likely to be employed (45.3% vs 29.0%, P=0.01), married (96.2 vs 86.6%, P=0.04) and from a higher HIV prevalence region (60.4% in Nyanza vs 28.8% in Nairobi vs, P <0.001). Among married women, those in a polygamous relationship were significantly more likely to seroconvert (19.6% vs 6.7%, P <0.001). Age, educational level, HIV risk perception, history of physical assault, and economic status were not associated with seroconversion. In multivariate analysis, region (OR3.6, 95%CI 2.1 to 6.4, P <0.001) and employment (OR1.9, 95%CI 1.1 to 3.3, P=0.03) independently predicted seroconversion.

Conclusions:  Repeat HIV testing in early postpartum was highly acceptable and resulted in detection of substantial HIV incidence. Women in regions with high HIV seroprevalence were at particularly high risk for seroconversion during pregnancy. Our findings highlight the urgent need for strategic approaches within PMTCT programs to prevent maternal HIV acquisition