Comparison of Pregnancy Outcomes and Vaginal Microbiota in Endometriosis Patients Undergoing Frozen Embryo Transfer Using Letrozole Combined HMG Versus Hormone Replacement Therapy with GnRH-a Pretreatment

This study investigated differences in reproductive outcomes and vaginal microbiota profiles between two endometrial preparation protocols—letrozole (LE) combined with human menopausal gonadotropin (HMG) and hormone replacement therapy (HRT) with GnRH-a pretreatment—in women with endometriosis (EMs) undergoing frozen embryo transfer (FET). Following 1∶1 propensity score matching, a total of 770 FET cycles were analyzed. No statistically significant differences were observed in live birth rates or clinical pregnancy rates between the two groups. However, the LE + HMG group showed a lower miscarriage trend (13.7% vs. 19.8%, P = 0.070) and significantly fewer cesarean deliveries (64.9% vs. 75.4%, P = 0.020) and hypertensive disorders of pregnancy (4.8% vs. 10.1%, P = 0.039). Recent evidence suggests that GnRH-a treatment may disrupt reproductive tract microbiota. Given ethical constraints on endometrial sampling during FET, vaginal microbiota was used as a surrogate to explore microbial differences between protocols. In the prospective arm, vaginal samples from 55 women in the LE + HMG group and 50 in the GnRH-a HRT group were analyzed using 16S rRNA sequencing and droplet digital PCR. While no significant differences were observed in Lactobacillus or Gardnerella abundance, the GnRH-a HRT group exhibited enrichment of potential pathogens, such as Escherichia-Shigella and Staphylococcus. In conclusion, although both protocols achieved comparable live birth outcomes, the LE + HMG regimen was associated with fewer obstetric complications and a more favorable vaginal microbiota profile compared to GnRH-a HRT.