Vaginal bacterial load in the second trimester is associated with early preterm birth recurrence: a nested case-control study

Objective: To assess the association between vaginal microbiome (VMB) composition and recurrent early spontaneous preterm birth (sPTB)/preterm prelabour rupture of membranes (PPROM). Design: Nested case-control study. Setting: UK tertiary referral hospital. Sample: High-risk women with previous sPTB/PPROM <34+0 weeks gestation who had a recurrence (n=22) or delivered at ≥37+0 weeks without PPROM (n=87). Methods: Vaginal swabs collected between 15-22 weeks gestation were analysed by 16S rRNA gene sequencing and 16S quantitative PCR. Main outcome measure: Recurrent early sPTB/PPROM. Results: 28/109 high-risk women had anaerobic vaginal dysbiosis, with the remainder dominated by lactobacilli (L. iners 36/109, L. crispatus 23/109, or other 22/109). VMB type, diversity, and stability were not associated with recurrence. Women with a recurrence, compared to those without, had a higher median vaginal bacterial load (8.64 vs. 7.89 log10 cells/μl, adjusted odds ratio (aOR)=1.90, 95% confidence interval (CI)=1.01-3.56, p=0.047) and estimated Lactobacillus concentration (8.59 vs. 7.48 log10 cells/μl, aOR=2.35, CI=1.20-4.61, p=0.013). A higher recurrence risk was associated with higher median bacterial loads for each VMB type after stratification, although statistical significance was reached only for L. iners-domination (aOR=3.44, CI=1.06-11.15, p=0.040). Women with anaerobic dysbiosis or L. iners-domination had a higher median vaginal bacterial load than women with a VMB dominated by L. crispatus or other lactobacilli (8.54, 7.96, 7.63, and 7.53 log10 cells/μl, respectively). Conclusions: Vaginal bacterial load is associated with early sPTB/PPROM recurrence. Domination by lactobacilli other than L. iners may protect women from developing high bacterial loads. Future PTB studies should quantify vaginal bacteria and yeasts.