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.
Different protocols for endometrial preparation in FET cycles, including natural cycles and hormone replacement therapy (HRT) cycles, create distinct hormonal environments that influence endometrial peristalsis and may impact pregnancy outcomes. Understanding how endometrial peristalsis varies by protocol and its effect on pregnancy outcomes is essential for optimizing IVF strategies.
Therefore, this study aims to evaluate endometrial peristalsis patterns in different FET protocols and their association with pregnancy outcomes.