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

Correlation Between Endometrial Peristalsis And Pregnancy Outcomes In Patients Undergoing Frozen Embryo Transfer With Hormone Replacement Therapy for Endometrial Preparation Protocol

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.

Frozen embryo transfer (FET) has become an integral component of assisted reproductive technology (ART), and successful implantation depends on adequate endometrial receptivity. Different endometrial preparation protocols, including natural cycles and hormone replacement therapy (HRT) cycles, create distinct hormonal environments that may influence endometrial physiology. Among these protocols, HRT is the most widely used approach because it is applicable to a broad range of patients and offers greater flexibility in treatment scheduling.

Endometrial peristalsis, characterized by rhythmic contractions of the uterine junctional zone, is thought to play an important role in embryo transport and implantation. Previous studies have suggested that the frequency and direction of endometrial peristalsis are influenced by ovarian steroid hormones and may be associated with implantation and pregnancy outcomes. However, most available evidence has focused on natural menstrual cycles, while data regarding endometrial peristalsis during HRT-prepared FET cycles remain limited and inconsistent. The temporal changes in endometrial peristalsis throughout HRT endometrial preparation and their relationship with reproductive outcomes have not been fully elucidated.

This prospective observational study is designed to characterize endometrial peristalsis during HRT-prepared FET cycles using transvaginal ultrasonography and to evaluate the association between endometrial peristalsis and pregnancy outcomes. Endometrial peristalsis will be assessed at predefined time points during endometrial preparation, and pregnancy outcomes will be compared according to the observed peristaltic patterns. The results of this study are expected to improve understanding of endometrial physiology during HRT-prepared FET cycles and provide evidence on the clinical significance of endometrial peristalsis for reproductive outcomes.

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