Vaginal Pathogenic Bacterial Colonization and Adverse Birth Outcomes among Pregnant Women Living with and without HIV in Tanzania

Background Both preterm birth and low birth weight (LBW) remain major contributors to neonatal morbidity and mortality in sub-Saharan Africa. Vaginal pathogenic bacterial colonization has been implicated in ascending genital tract infections, which may trigger these outcomes. However, there is a paucity of evidence on how vaginal bacterial colonization influences pregnancy outcomes, particularly in the context of maternal HIV infection. This study examined the associations between vaginal colonization by pathogenic bacteria and adverse birth outcomes (preterm birth and LBW) among pregnant women living with HIV and HIV-uninfected pregnant women in Mwanza city, Tanzania. Methods A hospital-based prospective analytical cohort study was conducted between August 2023 and February 2025 among pregnant women who delivered at select health facilities in Mwanza city. Eligible women with singleton pregnancies at ≥ 28 weeks gestation were consecutively recruited during the latent phase of labour. Lower vaginal swabs were collected and cultured via standard microbiological methods to identify pathogenic bacteria. Maternal demographic, behavioural, and clinical data were collected through structured questionnaires and clinical assessment tools. The primary outcomes were preterm birth (< 37 weeks) and LBW (< 2500 g). Logistic regression models stratified by HIV status were used to evaluate associations while adjusting for confounders. Results A total of 580 pregnant women were enrolled, 70.3% (n = 408) of whom had vaginal pathogenic bacterial colonization. Preterm birth occurred in 14.8% (n = 86) of deliveries, whereas 12.9% (n = 75) had LBW. Infants born to PWLWH had lower mean birth weights than those born to HUPWs did (2.9 kg, 95% CI: [2.8–3.0 kg] vs 3.1 [3.0–3.2] kg; p < 0.001). Overall, vaginal bacterial colonization was not significantly associated with preterm birth or LBW after adjustment for confounding factors among either HUPW (adjusted odds ratio (aOR) 1.13; 95% CI: 0.56–2.27) or PWLWH (aOR 0.76; 95% CI: 0.34–1.72). However, Escherichia coli demonstrated nonsignificant trends toward increased odds of LBW in both HUPW (OR = 2.48; 95% CI: 0.81–7.58; p = 0.11) and PWLWH (OR = 2.28; 95% CI: 0.75–6.95; p = 0.15). Vaginal Staphylococcus aureus showed a similar trend towards increased odds among PWLWH (OR = 2.76; 95% CI: 0.89–8.59; p = 0.08). Among the 12 (2.1%) ESBL-producing organisms, the odds of preterm delivery among HUPWs were as follows ([Crude odds ratio (COR)]: 11.6, [2.15–62.9; p = 0.004]). No increased odds were observed in PWLWH. Conclusion Overall, vaginal colonization was not independently associated with preterm birth or LBW after adjustment for confounders. However, infants of PWLWH had lower mean birth weights, and ESBL-producing bacteria among HUPW may be associated with preterm delivery. ESBL-producing bacteria may be critical and understudied contributors to preterm delivery. Longitudinal studies are needed to inform targeted screening and intervention strategies in pregnancy to address these potentially modifiable factors associated with preterm birth and LBW.