For IVF treatment, the process is broadly divided into three stages: ovulation promotion, in vitro fertilization and embryo culture, and embryo transfer. In terms of embryo transfer, there are fresh cycle transfer and frozen-thaw embryo transfer. In the former, embryos are transferred directly into the uterine cavity after egg retrieval, in vitro fertilization, and embryo culture. In the latter, the embryos are frozen and preserved after embryo culture using embryo freezing techniques. At a specific point in the menstrual cycle after the egg retrieval cycle, the embryos are thawed and revived and then transferred into the uterine cavity. So, which has a higher clinical pregnancy rate, fresh cycle embryo transfer or frozen-thawed embryo transfer? What are the advantages and disadvantages of each? Fresh cycle transfer, compared to freeze-thaw embryo transfer, has the following characteristics: 1. The embryos come directly from the in vitro culture system, without any additional disturbances such as embryo freezing and recovery, preserving the “original” developmental potential. In contrast, the freezing and resuscitation of embryos during the freeze-thaw embryo transfer cycle may cause damage to the ultrastructure or biomolecules (DNA, RNA, proteases, etc.) of the embryo, thus reducing the developmental potential of the embryo. But there is a more complicated side to this: because the freezing and thawing process is more or less harmful to the embryo, the embryo that survives this process is considered to be “stronger”. This means that the freezing and thawing of embryos actually plays a role in the selection of embryos. It is known that embryos that have been selected have a greater ability to implant and develop. In addition, with the widespread use of vitrification and resuscitation techniques, the damage to the embryo from the freezing and resuscitation process is close to zero. 2. There is a “natural”, 100% synchronization between the endometrium and embryo development. In contrast, the freeze-thaw embryo transfer cycle requires artificial control of the endometrial development rate or careful monitoring of the endometrial development to keep it in line with the development of the embryos to be recovered. This is obviously not so easy to do precisely. Of course, for a more specialized analysis, the situation may not be as straightforward as described above: the use of ovulation-promoting drugs during a fresh cycle may artificially alter the actual rate of development of the endometrium, so that instead the actual rate of development of the endometrium and the embryo during such a cycle does not match. 3. In fresh cycles, the function of the endometrium (to be precise, the embryo “tolerance” of the endometrium, that is, the ability of the endometrium to accept or induce embryo implantation) is impaired due to the effects of various ovulation-promoting or endocrine-regulating drugs. Freeze-thaw embryo transfer can completely circumvent this disadvantage. In summary, there are advantages and disadvantages to both embryo transfer strategies. In fact, different fertility centers around the world have different determinations of the final outcome of both in their respective clinical practices. However, the results of most centers (including the IX Reproductive Center) are that frozen-thawed embryo transfer is more effective than fresh embryo transfer. This is the reason why our center is now generally abandoning fresh embryo transfer (except for natural cycle egg retrieval).