Blastocyst, an embryo consisting of an inner cell mass, a blastocyst cavity, and a trophoblastic ectoderm, is formed when the fertilized egg is 5-6 days old. The blastocyst is the ultimate stage of embryonic culture in vitro and the stage at which the human embryo implants into the mother. Blastocyst transfer results in a higher implantation rate because blastocysts are more mature and better suited for growth in the uterine environment than oogenic embryos. Blastocyst scoring criteria: The formed blastocysts are graded according to the Garnder’s blastocyst grading method. Grade 1: early blastocyst, the volume of the blastocyst cavity is < half of the total blastocyst volume; Grade 2: the volume of the blastocyst cavity is > half of the total blastocyst volume; Grade 3: fully dilated blastocyst, the blastocyst cavity occupies the entire blastocyst; Grade 4: dilated blastocyst, the volume of the blastocyst cavity is significantly enlarged compared to the early blastocyst and the hyaline band is thinner; Grade 5: hatching blastocyst, the blastocyst is hatching from the rupture of the hyaline band; Grade 6: the blastocyst is hatching from the rupture of the hyaline band. Grade 5: hatching blastocysts, blastocysts are hatching from the rupture of the zona pellucida; Grade 6: hatching blastocysts, blastocysts are completely detached from the zona pellucida. Blastocysts of grades 3-6 are scored for endocytic mass (ICM) and trophoblastic ectoderm (TE) cells. The scoring of ICM is slightly different in each reproductive center, but in our center the scoring is as follows: Grade A, high number of cells, tightly bound; Grade B, low number of cells, still tightly bound; Grade C, low number of cells, loosely bound; Grade D, very low number of cells. Grade C, low cell number, loose epithelial cells; Grade D, very low cell number. Blastocysts with a score of ≥ 3CC on day 5 or 6 were classified as good quality blastocysts and were considered as freezable blastocysts. The culture of blastocysts is a process of further screening of embryos. More than half of the embryos developed by day 3 (oogenesis) are chromosomally abnormal. And after 5-6 days of in vitro culture, a part of the chromosomally abnormal embryos stalled their development or developed morphological abnormalities, and only a small number of embryos with good developmental potential formed beautiful blastocysts. In most laboratories in China, the morphologically best oogenesis embryos are preferentially frozen on the third day, and blastocysts are the embryos that win by further culturing the remaining embryos and eliminating the weak and obviously defective partners. So the formation of blastocysts is the result of the superiority of the embryo itself. Therefore, we need to look at blastocyst culture in a dialectical way, and the doctor will make the most reasonable choice of embryo transfer, freezing or further culturing according to the patient’s actual situation and after weighing many pros and cons.