Endometrial Peristalsis and Pregnancy Outcomes in Hormone Replacement Therapy (HRT) - Frozen Embryo Transfer (FET) Cycles

Endometrial peristalsis may influence embryo implantation and pregnancy outcomes, but its role during hormone replacement therapy (HRT)-prepared frozen embryo transfer (FET) cycles remains unclear. This prospective observational study will assess endometrial peristalsis at predefined time points during HRT-prepared FET cycles using transvaginal ultrasonography and evaluate its association with pregnancy outcomes. The study aims to clarify the clinical significance of endometrial peristalsis in HRT-prepared FET cycles and to provide evidence supporting endometrial assessment in assisted reproductive technology.

Inclusion Criteria: * Women aged 18 - 42 years old * Scheduled for frozen embryo transfer cycles using hormone replacement therapy protocol * Transferred no more than two cleavage embryos or one good-quality blastocyst or no more than two poor-quality blastocysts * Provision of written informed consent to participate Exclusion Criteria: * Having an allergy and contraindications for exogenous hormone administration (e.g., breast cancer, thromboembolic disease) * Cycles with preimplantation genetic testing, oocyte donation, or in vitro maturation * Having untreated uterine or adnexal abnormalities (e.g., intrauterine adhesions, unicornuate/ bicornuate/ arcuate uterus, endometrial polyp, large leiomyoma ≥5 cm in diameter, hydrosalpinx, endometrial hyperplasia) * Use of uterine relaxants or intralipid infusion during the embryo transfer process * Use of a GnRH-agonist for downregulation within one month