To investigate the effect of the number of embryo cells on the clinical outcome of frozen-thawed embryo transfer and explore the optimal policy for decreases of multiple pregnancy rate, patients who experienced day 3 vitrified double frozen-thawed embryo transfer were retrospectively analyzed. According to the number of embryonic cells in each pre-frozen embryo, the patients were divided into six groups: 8C2 (two 8-cell embryos), 8C1- < 8C1 (one 8-cell embryo and one under-8-cell embryo), 8C1- > 8C1 (one 8-cell embryo and one over-8-cell embryo), < 8C2 (two under-8-cell embryos), < 8C1- > 8C1 (one under-8-cell embryo and one over-8-cell embryo), and > 8C2 (two over-8-cell embryos). The clinical data were analyzed. The classification decision tree was used to analyze the optimal transfer strategy. A total of 2184 cycles of day 3 vitrified double frozen-thawed embryo transfer were enrolled. In day 3 double frozen-thawed embryo cycles, the 8C2 group and 8C1- > 8C1 group had significantly (P < 0.05) higher pregnancy and multiple pregnancy rates than the other groups. No significant (P > 0.05) difference existed in the pregnancy rate and live birth rate between the 8C1- < 8C1 group, 8C2 group and 8C1- > 8C1 group, but the implantation rate and multiple pregnancy rate in the 8C1- < 8C1 group were significantly (P < 0.05) lower than in the other two groups. Compared with the multiple pregnancy rate of all cycles, the cycles in two branches showed significantly (P < 0.05) higher multiple pregnancy rates (≤ 29 years old: 8C2 / 8C1- > 8C1; 29 < age ≤ 36 years for the first transfer: 8C2 / 8C1- < 8C1 / 8C1- > 8C1, one branch showed similar rate (≤ 29 years old: 8C2 / 8C1- > 8C1) for the first transfer, and the remaining four branches demonstrated significantly (P < 0.05) lower rates. The clinical pregnancy rates before and after optimization were 51.0% vs 50.5%, and the multiple pregnancy rates were 38.5% vs 16.9%. In conclusion, the number of pre-frozen embryonic cells is an important factor affecting the clinical outcome of frozen-thawed embryo transfer in day 3 double good embryos frozen-thawed cycles. The age of patient, number of embryo cells, and the first time of transfer are the most valuable parameters for prediction. For women ≤ 29 years old, the single embryo transfer (SET) strategy was to choose an embryo ≥ 8 cells, and for women with < 29 age ≤ 36 years old, the SET strategy in the first transfer was to choose an embryo ≥ 8 cells.
Keywords: Birth rate; Embryonic cell numbers; Frozen-thawed embryo transfer; Morphological score; Multiple pregnancy rate.