I. Oogenesis embryos At present, morphological scoring methods are commonly used to grade embryos at home and abroad. The fetal scoring method used in our center firstly records the number of cells (oocytes) of the embryo, and then grades the embryos according to the degree of homogeneity of oocyte size, whether the morphology is regular or not, and the number of fragments, which are divided into grade I, grade II, grade III and grade IV in order. There is no relationship between the level of grade and the number of cells, therefore, we can see 4-cell grade I embryos and also 8-cell grade I embryos. The following pictures are examples of grade I to IV embryos in order. The first picture (top left) is an 8-cell grade I embryo, abbreviated as “8Ⅰ”. The second picture (top right) is an 8-cell grade II embryo, abbreviated as “8II”. The order of the grades is I > II > III > IV. In addition, the developmental speed (i.e., cell count) of the embryo is also critical. The ideal embryo developmental speed should be 6-8 cells on the third day after egg retrieval. As the embryo cell count and grade decrease, the embryo developmental potential decreases and the chances of pregnancy after transfer decrease. Currently, the Gardner scoring method is commonly used to score blastocysts, which is based on the size of the blastocyst cavity and is divided into stages 1-6, and then graded according to the number of inner cell masses and trophoblast cells. For example, for a blastocyst with a score of “4AB”, “4” means the blastocyst is at stage 4, “A” means the blastocyst’s inner cell mass rating, “B ” indicates the rating of the trophoblast layer of the blastocyst. Where “A” is better than “B” and “B” is better than “C”. Therefore, the same stage 4 blastocyst is graded 4AA > 4AB > 4BB > 4BC > 4CC. The images below are examples of blastocysts. The first picture (left) shows a stage 1 blastocyst, the second picture (middle) shows a stage 4 blastocyst with the grade “4AB”, and the third picture (right) shows a stage 5 blastocyst with the grade “5AA”. At present, our center freezes or transfers blastocysts with stage 4-6, inner cell mass rating ≥ B, and trophoblast layer rating ≥ C, i.e. rating of 4BC and above.