In recent years, with the development of frozen embryo technology, it is often necessary to freeze embryos for frozen embryo transfer depending on the clinical situation. Luteinizing support, progesterone for intramuscular injection, is often used in the frozen embryo transfer cycle and is very widely used. Some patients have discovered why progesterone is given three days in advance for day 3 frozen embryos, while frozen blastocysts are given five days. To be precise, as long as the embryos are frozen, they can be called frozen embryos, including the frozen embryos at the 3rd day of development and the frozen blastocysts at the 5th or 6th day of development. The term “frozen embryo” in this question should specifically refer to the “day 3 oogenesis embryo” and should be distinguished from the “day 5 or 6 blastocyst”. We know that farmers plant cotton in spring and wheat in autumn. Different seeds need different land at different times of the year. These two types of embryos have different days of development, which also determines the number of days of endosperm required for them. The focus of frozen embryo transfer is on the preparation of the endometrium. First of all, we still need to see that under natural conditions, after ovulation, the egg is fertilized to form embryos developing day by day; at the same time, the follicle wall after ovulation will form corpus luteum, which has the function of secreting progesterone, and the concentration of progesterone in the body increases, which acts on the endometrium day by day, allowing the endometrium to metastasize day by day and enter the window period of embryo implantation, which facilitates the embryo to implant in the endometrium; after that, progesterone further acts to promote After that, progesterone acts further to promote the development of maternal-fetal interface, placenta formation and fetal growth and development. The synchronization of embryo development and endometrial development is one of the key elements of the embryo implantation process. Frozen embryo cycles include natural and artificial cycles. Natural cycles have their own follicle development and secrete endogenous progesterone, which can be used with little or no luteinizing support. However, luteal support is indispensable in the artificial cycle, relying on daily medication that mimics the day-by-day transforming effect of the natural cycle on the endometrium. The longer the number of days, the further back the metaphase of the endometrium progresses, and the window period for endometrial implantation will first open and then close, and after closing it will not easily accept embryo implantation. Both preparation protocols, require that progesterone shots can only be started three days earlier for third day frozen embryos, while blastocysts require progesterone shots five days earlier to create synchronization of endometrial and embryonic development that is most favorable for embryo implantation. Not only in frozen embryo transfer, but also in fresh embryo transfer cycle, oogenesis embryos developed to day 3 are transferred on the day of egg retrieval plus three days, while blastocysts are usually transferred on the fifth day after the day of egg retrieval, both for the purpose of creating synchronization between embryo development and endometrial development. In frozen embryo transfer cycles, some patients with repeated transfer failures may have early and pushed back endometrial implantation windows, and foreign studies have reported the use of specific endometrial tolerance tests to aid in endometrial tolerance determination, personalize the number of days of proposed transfer, and improve pregnancy rates. The medications used for luteal support include not only injectable progesterone, but also transvaginal gels, vaginal suppositories, oral medications, and other drugs. The number of days of luteal support, the type of medication, and the dosage all need to be designed by a specialist to best suit the patient’s individual situation and to adjust the luteal support program in stages.