Received January 30, 2022
Accepted August 01, 2022

Objective: The role of Lactobacillus-dominant microbiota in the endometrium on reproductive function is unclear. We aimed to explore prospectively the impact of the balance of Lactobacillus and pathological bacteria in the endometrial and vaginal microbiomes on pregnancy outcomes for women using assisted reproductive technology (ART).　 Methods: This study included 35 women with infertility who underwent good-quality embryo transfer. The cutoff values of abundance in Lactobacillus species (spp.) and pathological bacteria in the endometrium and vagina were calculated. Women with Lactobacillus spp. and pathological bacteria abundance above the cutoff values were categorized in the high-abundance group, whereas those with abundance below the cutoff values, in the low abundance group, respectively. We further divided the patients into four groups based on the combination of high/low abundance of Lactobacillus spp. and pathogens. Results: Of the 35 cases of good-quality embryo transfer, 21 pregnancies were achieved. The group with high abundance of Lactobacillus spp. and low abundance of pathological bacteria belonged to pregnant cases with a significantly higher rate, whereas those the opposite combination groups belonged to nonpregnant cases with a significantly higher rate (P=0.022). Conclusion: The balance between Lactobacillus and pathological bacteria occupancies in the endometrial and vaginal microbiomes is associated with pregnancy using ART.