Association Between Peri-Transfer Endometrial Peristalsis And Live Birth In Modified Natural Cycle Frozen Embryo Transfer: A Prospective Cohort Study

Effects Of Modified Natural Cycle Protocol Peristalsis And Live Birth In Frozen Embryo Transfer: A Prospective Cohort Study

The uterus is a dynamic muscular organ that undergoes rhythmic, wave-like contractions known as endometrial peristalsis or endometrial waves. This muscular activity, which is an essential component of natural fertility, presents a nuanced and sometimes contradictory role in the context of assisted reproductive treatments. Endometrial peristalsis refers to the frequency, amplitude, and pattern of myometrial contractions occurring in different reproductive phases. These peristalsis play vital roles in sperm transport, embryo migration, and implantation. Clinical and imaging studies suggest that abnormal patterns or excessive contractility at the time of embryo transfer may disrupt endometrial-embryo synchrony, impair implantation, and increase miscarriage risk. However, most evidence on endometrial peristalsis pertains to fresh embryo transfer cycles, natural conceptions, or pathological contexts, such as adenomyosis or fibroids, with limited insights regarding its effects on different endometrial preparation protocols in frozen embryo transfer (FET). Understanding the dynamics of endometrial peristalsis in this context is clinically important, as inappropriate contractile activity could physically expel the embryo or create a non-receptive environment, ultimately reducing the chances of live birth. Despite its theoretical significance, there is a paucity of robust, prospective data correlating endometrial peristalsis patterns measured around the time of FET with different endometrial preparation protocols with subsequent pregnancy outcomes.

The uterus is a dynamic muscular organ that undergoes rhythmic, wave-like contractions known as endometrial peristalsis or endometrial waves. This muscular activity, which is an essential component of natural fertility, presents a nuanced and sometimes contradictory role in the context of assisted reproductive treatments. Endometrial peristalsis refers to the frequency, amplitude, and pattern of myometrial contractions occurring in different reproductive phases. These peristalsis play vital roles in sperm transport, embryo migration, and implantation.

Clinical and imaging studies suggest that abnormal patterns or excessive contractility at the time of embryo transfer may disrupt endometrial-embryo synchrony, impair implantation, and increase miscarriage risk. However, most evidence on uterine contractility pertains to fresh embryo transfer cycles, natural conceptions, or pathological contexts, such as adenomyosis or fibroids, with limited insights regarding its effects on different FET protocols. Several studies have demonstrated an inverse relationship between endometrial peristalsis and IVF success. Masroor et al. found that patients with lower endometrial peristaltic wave frequency (<4 waves/min) before embryo transfer had significantly higher chances of clinical pregnancy and live birth compared to those with more frequent peristalsis. Similarly, Chung et al. reported that increased endometrial peristalsis frequency immediately after embryo transfer was linked to reduced live birth rates, suggesting that excessive motility may physically expel the embryo or disturb its implantation. In the prospective cohort study of 292 infertile women, Zhu et al. found that lower uterine peristaltic wave frequency (<3.0 waves/min) before embryo transfer is associated with higher clinical pregnancy rates in both fresh and frozen-thawed embryo transfer cycles. In a study by Vuong et al. on patients with repeated implantation failure, they found that administering atosiban to patients with uterine peristalsis exceeding 16 waves per 4 minutes could improve pregnancy rates.

In recent years, there has been growing interest in performing FET in natural cycles. Natural-cycle FET protocols may better reproduce physiological endocrine and uterine environments than fully artificial, hormone-replacement cycles, potentially influencing uterine contractility patterns, implantation rates, and obstetric outcomes, while also reducing medication burden for ovulatory women. Building on this evidence, the present study will examine the relationship between peri-transfer uterine peristalsis in natural-cycle frozen embryo transfer and subsequent live birth, with the aim of identifying clinically relevant thresholds of contractile activity that could inform individualized embryo-transfer strategies.

Inclusion Criteria:

• Women aged 18 - 42 years old
• Scheduled for frozen embryo transfer cycles using hormone replacement therapy protocol or natural cycle protocol (True natural cycles or modified natural cycles)
• Transferred no more than two cleavage embryos or one good-quality blastocyst or no more than two poor-quality blastocysts

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.
• PCOS patients