Number of Blastomeres in Day-2 Embryos Affect the Rates of Blastocyst Formation and Clinical Pregnancy During In Vitro Fertilization Cycles

In the current in vitro fertilization and embryo transfer protocol, the 8 blastomeres in the day-3 embryo are selected for transfer because these embryos can produce high rates of blastocyst formation and clinical pregnancy. However, the relationship between the blastomere number in day-2 embryos and the rate of blastocyst formation or clinical pregnancy remains unclear. The purpose of this retrospective study is to explore the relationship between the blastomere number in day-2 embryos and the rate of blastocyst formation or clinical pregnancy. From January 2015 to April 2020, we collected 8126 day-3 embryos (8 blastomeres) from 2282 patients. These embryos were classified into 8 groups (1 blastomere, 2 blastomeres, 3 blastomeres, 4 blastomeres, 5 blastomeres, 6 blastomeres, 7 blastomeres, and 8 blastomeres) based on their blastomeres number on day 2 after insemination. Of these groups, the 4 blastomeres group accounted for the largest proportion (74.44%). The 1 blastomere group accounted for the smallest proportion (0.22%). A total of 3554 day-3 embryos (8 blastomeres) from 1648 patients developed into blastocysts. The rate of blastocyst formation from the 4 blastomeres group was the highest (94.06%). Finally, 800 patients received single day-3 embryos (8 blastomeres) transfer. The rate of clinical pregnancy from 4 blastomeres group was the highest (51.98%). In conclusion, our data provide evidence that the number of blastomeres in day-2 embryos affects the rate of blastocyst formation and clinical pregnancy.