Comparison of pregnancy outcomes and vaginal microbiota in women with endometriosis undergoing frozen embryo transfer using letrozole combined with 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 gonadotropin-releasing hormone agonist (GnRH-a) pretreatment—in women with endometriosis 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 nonsignificant trend toward a lower miscarriage rate (13.7% vs. 19.8%, P = 0.070), as well as lower observed rates of cesarean delivery (64.9% vs. 75.4%, P = 0.020) and hypertensive disorders of pregnancy (4.8% vs. 10.1%, P = 0.039). Previous evidence suggests an association between GnRH-a treatment and reproductive tract microbiota composition. Given the clinical and ethical constraints of endometrial sampling during FET, vaginal microbiota was evaluated as a non-invasive indicator of lower reproductive tract microbial composition. 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 showed higher relative abundances of several low-abundance taxa, such as Escherichia-Shigella, Limosilactobacillus, and Staphylococcus. In conclusion, although both protocols achieved comparable live birth outcomes, the LE + HMG regimen was associated with lower rates of cesarean delivery and hypertensive disorders of pregnancy, while the two protocols exhibited differences in several low-abundance vaginal bacterial taxa. However, longitudinal studies incorporating baseline and repeated sampling are needed to clarify temporal relationships and causality.