Conclusion
In summary, although the results of this study need to be confirmed in multicenter randomized trials, they suggest that obtaining as many high-quality blastocysts as possible and programming single blastocyst transplantation deserved consideration. Although the optimal duration of progesterone supplementation remains to be clarified, it is recommended to transfer embryos on days 3 and 5 with a degree of flexibility. However, there may be different and possibly narrower WOI for the blastocyst on the sixth day of vitrification. Further research is required to optimize FET protocols and distinguish other contributing factors that may affect pregnancy outcomes after transferring frozen-thawed embryos.