Due to progress of embryo vitrification in recent years, frozen embryo transfer (FET) of in vitro fertilization (IVF) has already been a world trend. The most used two protocols of FET are natural cycles (NCs) and artificial cycles (ACs). Recent studies showed that IVF outcomes of NCs were superior to ACs. We hypothesized that the result may be because of better endometrial receptivity of NCs compared to ACs. In NCs, no hormonal agents were administrated. Thus, endometrial receptivity was not disturbed by hormone and became better. However, there were few studies to explore the issue. Therefore, the aim of this study was to investigate the expression of endometrium during window of implantation (WOI) between NCs and ACs.~The study will be performed from July 1st, 2022 to December 31th, 2023 in our reproductive center. We plan to enroll 60 infertile women who have underwent IVF cycles and would like to receive endometrial receptivity assay. We divide the participants into two groups, NCs and ACs. In NCs, no hormonal agents will be used. We identify ovulation day via serial hormone data and sample endometrium during WOI (LH surge + 7 days). In ACs, first, estrogen will be given for 10~14 days and then transvaginal sonography will be conducted. When endometrial thickness reaches 8 mm at least, progesterone will be prescribed. Endometrium is sampled during WOI (P4 + 5 days). We will record basic characteristics, infertility history, hormone data, ovarian reserve and endometrial thickness of infertile women in both groups. Endometrial samples will be assessed miRNA, implantation factors, uterine NK cells and T cells in both groups.
The aim of this study was to investigate the expression of endometrium during window of implantation between natural cycles and artificial cycles.