Vaginal microbiome and preterm birth: Composition, mechanisms and microbiota‑directed therapies (Review).

Preterm birth (PTB) is a global maternal and neonatal health challenge, affecting ~15 million infants each year. Despite advances in obstetric and neonatal care, PTB‑related morbidity and mortality remain high. Emerging evidence implicates dysbiosis of the vaginal microbiota (VMB) as a key contributor to PTB. A healthy VMB is typically dominated by Lactobacillus spp., which maintain an acidic vaginal environment and inhibit pathogen colonization. Conversely, reduced Lactobacilli abundance alongside overgrowth of anaerobic taxa such as Gardnerella, Atopobium and Mycoplasma is strongly associated with spontaneous PTB and preterm premature rupture of membranes. Excessive proliferation of vaginal pathogens may lead to ascending infection and intra‑amniotic inflammation via activation of host Toll‑like receptor signaling and induction of pro‑inflammatory cytokines IL‑1β, IL‑6 and IL‑8. Moreover, VMB‑derived metabolites such as lactate play important roles in immunomodulation and inflammation. Although antibiotics remain the mainstay for treating bacterial vaginosis, their non‑specific effects often disrupt microbial balance and predispose to recurrence. Recently, probiotic therapies and VMB transplantation have emerged as promising alternative or adjunctive strategies for PTB prevention and management. However, variability in probiotic efficacy and lack of standardized intervention protocols remain significant challenges. The present review examined pregnancy‑associated VMB dynamics, the mechanisms linking dysbiosis to PTB risk and future microbiome‑based intervention strategies, with the aim of informing theoretical and practical approaches to reduce the global burden of preterm birth.