Abstract
OBJECTIVE: To investigate whether vaginal and endometrial microbiota impact pregnancy outcomes in frozen embryo transfers (FET) during hormone replacement cycles. METHODS: This retrospective cohort study analyzed 51 first-time FET patients from January 2021 to December 2022. Using 16 S rRNA sequencing, patients were stratified based on uterine microbiota composition into Lactobacillus-dominant (LD, 20 cases) and non-Lactobacillus-dominant (NLD, 31 cases) groups based on uterine microbiota. Vaginal (A1, A2) and uterine (B1, B2) microbiota were compared, along with pregnancy outcomes. RESULTS: Sequencing yielded 5,753,727 valid sequences and 1,545 OTUs. Alpha diversity showed lower Shannon index (P=0.034) and higher Simpson index in B1 vs. B2 (P=0.017), indicating reduced diversity in B1. Beta diversity analysis revealed more concentrated and similar microbial composition in B1, with significant differences between B1 and B2 (Anosim R = 0.3812, P = 0.001). B1 had higher abundance of Firmicutes and Lactobacillus, while B2 had more Proteobacteria and potential pathogens. Clinical pregnancy rate (75.00%) and live birth rate (65.00%) were higher in LD compared to NLD (45.16% and 29.03%, respectively, P = 0.036 and P = 0.011). CONCLUSION: Vaginal microbiota do not fully reflect uterine microbiota. Dominance of Lactobacillus in the uterine microbiome is beneficial for favorable FET outcomes, while the presence of non-Lactobacillus dominant species may negatively impact FET results.