The vaginal microbiome and the risk of preterm birth: a systematic review and network meta-analysis
Comprehensive Overview
This study, published in Scientific Reports (2022), investigated whether the community of bacteria naturally living in the vagina, known as the vaginal microbiome, is linked to the risk of giving birth prematurely, a major cause of newborn illness and death that affects more than 1 in 10 births worldwide. Prior research had suggested that a vaginal environment dominated by protective Lactobacillus bacteria is generally healthier, while a more diverse mix with fewer of these protective bacteria, often overlapping with a common imbalance called bacterial vaginosis, might raise infection risk, but individual studies on this link had produced inconsistent results. Researchers systematically reviewed the medical literature and combined data from 17 eligible studies published between 2014 and 2021, together covering more than 2,500 pregnancies, using a statistical technique called network meta-analysis that allowed them to directly and indirectly compare five different vaginal bacterial community types simultaneously, rather than comparing just two groups at a time as most previous studies had done. Pregnant women were grouped according to which type of bacteria dominated their vaginal microbiome, and researchers then tracked how often women in each group went on to deliver prematurely, comparing the results across all five groups at once.
Outcomes and Implications
The most notable finding was that women with a vaginal microbiome lacking substantial protective Lactobacillus bacteria, termed "low-lactobacilli," were significantly more likely to deliver prematurely, facing roughly 70 percent higher odds of preterm birth compared with women whose vaginal microbiome was dominated by a specific protective species, Lactobacillus crispatus. Among women who ultimately delivered preterm, about 4 in 10 had this low-lactobacilli pattern, compared with roughly 3 in 10 among women who delivered at full term. Women dominated by a different Lactobacillus species, L. jensenii, also trended toward higher risk, though this result fell just short of statistical significance. Across the five bacterial community types studied, L. crispatus dominance consistently emerged as the most protective pattern, while low-lactobacilli and L. jensenii dominance trended toward the least favorable outcomes. These patterns held up consistently across several subgroup analyses, including when researchers focused specifically on spontaneous preterm labor and when comparing different geographic regions. This matters because the vaginal microbiome is a modifiable, measurable factor that could eventually help identify pregnant women at elevated risk and inform future prevention strategies, such as interventions aimed at restoring a more protective bacterial balance. The authors note that many included studies were limited by small sample sizes and methodological differences in how bacteria were measured and categorized. The authors conclude that vaginal microbiome diversity appears to meaningfully influence preterm birth risk, and that further well-designed longitudinal research is needed to clarify the underlying biological mechanisms.