Does HIV infection modify the relationship between pre-pregnancy body mass index and adverse birth outcomes?

Background South Africa faces dual epidemics of HIV and obesity; however, little research has explored whether HIV status influences associations between pre-pregnancy body mass index (BMI) and adverse birth outcomes. Objectives To examine associations between pre-pregnancy body mass index (BMI) and adverse birth outcomes, and if they differ by HIV status. Methods We followed HIV-uninfected and -infected pregnant women initiating antiretroviral therapy (ART) from first antenatal visit through delivery. HIV-infected women initiated ART (tenofovir-emtricitabine/lamivudine-efavirenz) in pregnancy. Estimated pre-pregnancy BMI (kg/m(2)) was categorised as underweight (<18.5), normal (18.5-24.9), overweight (25.0-29.9), and obese (>= 30.0). We used modified Poisson regression to estimate risk ratios (RR) for associations between pre-pregnancy BMI and adverse birth outcomes and explored modification by HIV status. Results Among 1116 women (53% HIV-infected), 44% of HIV-uninfected women and 36% of HIV-infected women were classified as obese; 4% of women were underweight. Overall, 12% of infants were delivered preterm (<37 weeks), 10% small for gestational age (SGA, <10th percentile), and 9% large for gestational age (LGA, >90th percentile). Compared to HIV-uninfected women, HIV-infected women on ART had less LGA (5% vs 13%) but more SGA (12% vs 8%), and a similar proportion of preterm (13% vs 11%) infants. Pre-pregnancy BMI was not associated with preterm birth. Among HIV-uninfected women, obesity modestly increased the risk of LGA (RR 1.34, 95% confidence interval [CI] 0.82, 2.19), and underweight modestly elevated the risk of SGA (RR 1.66, 95% CI 0.79, 3.46). These associations were attenuated among HIV-infected women (RR 1.07, 95% CI 0.44, 2.64 for LGA, and RR 1.34, 95% CI 0.49, 3.64 for SGA). Conclusions In this urban African setting of high HIV prevalence, pre-pregnancy obesity was common and did not vary by HIV status. In HIV-uninfected women, obesity increased the risk of LGA and being underweight the risk of SGA, compared with among HIV-uninfected women.