In recent years, with the significant improvement of ovarian stimulation regimens, embryo cryopreservation and thawing techniques, the frozen-thawed embryo transfer (FET) procedure has gained more popularity in assisted reproductive technology (ART) than fresh embryo transfer. In general, the success of FET depends on the quality of embryo, receptivity of uterine, and synchronization between embryo and endometrium. Therefore, it is of particular importance to determine the optimal timing to perform FET, known as the "window of implantation".