As the conditions for embryo culture in assisted reproductive technology continue to improve, embryo transfer strategies also evolve, beginning with the first early embryo transfer and progressing to blastocyst transfer, and then the sequential transfer strategy was derived, which is presented in this work. The sequential transplantation was originally intended to increase embryo implantation rates in patients who had previously failed repeated implantation attempts. Synchronization of embryo implantation and endometrial receptivity is the key to improving the implantation pregnancy rate. The ‘implant window period’ of the endometrium of different patients is not entirely consistent, and it may be opened in advance or delayed. Studies have shown that the ‘implantation window’ opening time may vary from cycle to cycle in the same patient. Approximately one-third of infertility patients experience changes in the endometrial implantation window, which is manifested as the shortening, advance, or delay . Sequential transplantation allows the embryo to be placed in the uterine cavity twice in one implantation process, increasing the chances of the embryo and endometrial “implantation window” synchronization and thus improving the patient’s pregnancy rate. The mechanism involves the transplantation stimulation enhances the sensitivity of the endometrium as first step, which causes endometrial cells to secrete cytokines such as IL-1, transforming growth factor, epidermal growth factor, and granulocyte-macrophage stimulation factor, which seeds the endometrium to enter the “planting window” state and improves the transplantation outcomes[5, 7].
This study showed that sequential transfer had a higher clinical pregnancy and embryo implantation rate than early embryo transfer cycle but had no advantage over the blastocyst transfer cycle. The results were consistent with the findings of Wang Miaomiao’s et al. research[8, 9]. Sequential transplantation required at least two embryos per cycle, significantly higher than the blastocyst transfer cycle, but it did not improve clinical outcomes. Due to the low utilization rate of embryos in the sequential transplantation cycle, the sequential transplantation strategy should be carefully used for patients undergoing the first transplant to avoid embryo waste . Meanwhile, sequential migration is still controversial. Machtinger et al.  hypothesized that two-step transplantation could improve embryo implantation and clinical pregnancy rates in patients with repeated implantation failure . Fang et al. . pointed out in their study that sequential transplantation could significantly improve clinical pregnancy and embryo implantation rates in patients with repeated implantation failure compared to embryo transfer at the cleavage stage. However, Jacob Ashkenazi et al.  believed that when compared to traditional embryo transfer at the cleavage stage, two-step transplantation is not conducive to improving embryo implantation and clinical pregnancy rate, which could be due to repeated intrauterine operations that may lead to infection and endometrial damage, increasing the risk of abortion. However, Tur-Kaspa et al.  showed that sequential transplantation did not increase the risk of infection. Some researchers compared the rate of early abortion after blastocyst and cleavage embryo transplantation. It concluded that blastocyst transfer has a lower risk of early abortion than embryo transfer in both the egg and freeze-thaw cycle stages, which did not appeared in our data. There remains a scarcity of multi-center, large-sample data that can be used to confirm the feasibility of sequential transplantation. The existing controversies also suggest that close attention should be paid to the application indications of sequential transplantation strategies to achieve greater social benefits.
In conclusion, sequential transplantation is an emerging strategy for embryo transfer. By increasing the number of transplants and the number of embryos in the cycle, the ideal pregnancy rate, and the number of endometrial transplant preparations for infertile patients can be reduced, as well as the economic and time costs of patients. Simultaneously, more clinical evidence is needed to support the study of birth outcomes and the subsequent development of the sequential transplantation cycle. In future studies, it is necessary to comprehensively evaluate various factors of patients, and try to combine with immune indicators to classify and stratified detection of patients, so as to further clarify the mechanism of sequential transplantation, so as to accurately target populations and give full play to the advantages of sequential transplantation.