Abstract:Abstract: Objective To investigate the optimal time of endometrial receptivity in hormone replacement therapy-frozen embryo transfer (HRT-FET) cycles. Methods According to days of exposure to progesterone, 52 cases undergoing HRT-FET were divided into P3 group, P4 group, P5 group, P6 group and P7 group. Pinopode morphology, coverage area, integrin alpha v beta 3 (αvβ3), homeobox gene A10 (HOXA10) and leukemia inhibitory factor (LIF) were detected and compared among the groups. Results Most of the cases had the highest expression of αvβ3, HOXA10 and LIF after 6-day exposure to progesterone. Conclusions It is feasible to evaluate the degree of sporadic development of embryos as embryo transfer time. The transplantation of the embryo on the 3rd day of progesterone supplementation may be an ideal choice for the synchronization of endometrium and embryo.