After obtaining embryos in an ovulation promotion cycle, most patients freeze the embryos and transfer them after another cycle with embryo recovery. Why do we do this? First, the quality of the frozen and thawed embryos. We use a vitrification freezing technique that allows the embryos to form a solid state during the rapid cooling process, preventing the formation of ice crystals of water within the embryos and causing minimal damage to the quality of the embryos. Secondly, due to different medication regimens, the level of estrogen and progesterone in the body is different from the physiological state, which has different effects on the tolerance of the endometrium and may reduce the chance of embryo implantation. In contrast, during the frozen embryo transfer cycle, the hormone level in the body is close to the physiological state, which is more favorable for embryo implantation. Third, some patients with high estrogen levels who transfer embryos in a fresh cycle are at risk of ovarian hyperstimulation syndrome, which may be very severe and even life-threatening if pregnancy occurs, so it is not recommended to take the risk of transferring fresh embryos. Fourth, current studies have found that babies born in frozen embryo transfer cycles have greater birth weight and height than babies born in fresh cycle transfers, so we prefer to transfer frozen-thawed embryos from the perspective of baby health.