Abstract: Objective To investigate how to optimize the protocol of embryo cryopreservation to improve the success of frozen-thawed embryo transfer (FET), reduce multiple pregnancy rate and increase the cumulative pregnancy rate from one oocyte retrieval process. Methods The clinical data of 1 166 FET cycles were retrospectively analyzed and separated into different groups： 445 for vitrification and 721 for slow-freezing. The vitrification group was divided into single embryo (28 cy⁃ cles), double embryos (71 cycles) and triple embryos (346 cycles). 0-1 optimal embryo was called O0-1 group (235 cycles), 2 optimal embryos were called O2 group (80 cycles), 3 optimal embryos were called O3 group (130 cycles). The difference preg⁃ nancy outcomes (implantation rate, clinical pregnancy, abortion rate and live- birth rate) were compared between groups. Results (1) There were significantly higher embryo survival rate（98.3% vs 73.1%), embryo recovery rate without damaging （83.3% vs 62.1%), implantation rate（36.8% vs 29.9%), clinical pregnancy（57.1% vs 44.0%) and live-birth rate（47.9% vs 34.5%) in vitrification group than those of slow freezing group（P＜0.05). (2) There were significantly higher clinical pregnan⁃ cy rate（62.7% vs 39.4% vs 32.1%), multiple pregnancy rate（37.6% vs 10.7% vs 0%） and live-birth rate（52.6% vs 31.0% vs 21.4%）in triple embryos group than those of the single and double embryos groups (P＜0.05). (3) There were significantly higher implantation rate（33.0% vs 27.1%), clinical pregnancy（65.2% vs 48.1%), multiple pregnancy rate（38.0% vs 20.4%) and live-birth rate（56.2% vs 39.2%) in O2-3 group than those of O0-1 group（P＜0.05). (4) There were significantly higher im⁃ plantation rate（36.8% vs 26.3%) and multiple pregnancy rate (46.7% vs 21.3%) in O3 group than those of O2 group (P＜0.05). There were no significant differences in clinical pregnancy（69.2% vs 58.8%), abortion rate（15.6% vs 10.6%), ectopic pregnancy rate（0 vs 0), live-birth rate（58.5% vs 52.5%) and preterm birth rate（26.3% vs 14.3%) between two groups (P＞0.05). Conclusion Vitrification technology can improve the clinical pregnancy and live-birth rate, and decrease multiple preg⁃ nancy rate. Two optimal embryos in one tube are supposed to be the preferred method for embryo cryopreservation.

Key words: frozen-thawed embryo transfer(FET), vitrification, slow-freezing, optimal embryo, cumulative pregnancy rate