As embryo freezing and resuscitation techniques have entered the era of vitrification and freezing, embryo freezing and resuscitation is no longer a significant impediment to embryo developmental potential. In this context, to circumvent the negative effects of ovulation promotion protocols on the endometrium, physicians are increasingly opting to perform embryo freezing for egg retrieval cycles and subsequent freeze-thaw embryo transfer. For frozen-thaw embryo transfer, the key to success is the developmental potential of the embryo, the function and status of the endometrium, and the synchronization of development between the two. To achieve the latter two, the morphology and the degree of development of the endometrium need to be monitored and intervened. There are three broad clinical treatment options used for this purpose: 1) natural cycles. 2) ovulation-promoting cycles. 3) hormone replacement cycles. Natural cycles: The advantage of natural cycles is that they are “natural”, avoiding the possible effects of various drugs on the endometrial function and usually do not require complicated artificial corpus luteum support after embryo transfer due to the presence of a normal corpus luteum after ovulation. In particular, the classical natural cycle does not require medication, is simple and easy to perform, and is easily accepted by patients. However, the classical natural cycle also has the disadvantage that it is often difficult to capture its exact timing of ovulation and to determine easily the rate of development of the endometrium in order to match the degree of development of the resuscitated embryo. This is sometimes compensated for by the use of artificial ovulation induction. If ovulation is induced artificially, the cycle is called a “modified natural cycle”. In addition, the use of natural cycles for endometrial preparation presupposes that the patient has regular ovulation; otherwise, one of the following two types of preparation is used: Ovulation-promoting cycles: Ovulation-promoting drugs are used to stimulate follicular growth, which leads to the development of the endometrium and eventually induces ovulation. This approach is suitable for patients with ovulation disorders that prevent them from doing natural cycles. Also, because ovulation promotion often causes multiple follicles to develop, estrogen levels will be higher than natural levels, which may help some patients with endometrial hyperplasia. However, ovulation-promoting drugs often require daily injections, causing some pain and inconvenience to the patient. They may also impair endometrial tolerance once the estrogen levels are too high due to multiple follicle development. Hormone replacement cycle: Using exogenous estrogen and progestin to artificially simulate the physiological hormone fluctuation pattern in the natural menstrual cycle, artificially induce endometrial development and control the point in time when the endometrium changes from proliferation to secretion. It allows for more accurate synchronization of endometrial-embryonic developmental rates than both ovulation-promoting cycles and natural cycles. However, this method appears to be cumbersome in treatment because estrogen is used early in menstruation, which inhibits follicular development, there is no natural luteal function throughout the treatment cycle, and prolonged, high-dose luteal drug support is required after embryo transfer. In addition, some patients still have follicular development after such therapy, which may create problems for artificial regulation of the endometrium. Therefore, pituitary descending regulation followed by hormone replacement is sometimes used. However, this is much more cumbersome and can cause some degree of perturbation to the patient’s recent endocrine profile. In my personal experience, this is usually not the preferred approach to hormone replacement cycles.