Under normal circumstances, if a woman has one blastocyst transfer, mainly for women who have delivered a healthy baby, had a cesarean section with a scarred uterus or had a fibroid excision, one blastocyst is good for the normal growth and development of the embryo, but if two blastocysts are transferred, it will easily lead to a twin pregnancy, and the excessive enlargement of the uterus later will lead to a high risk of pre-rupture, uterine rupture and threaten the life and health of the woman and the fetus. In addition, twin pregnancies can also increase the risk of pregnancy complications. Therefore, before an IVF transfer, a woman’s doctor will make a choice based on her fertility status and health condition. In order to increase the success rate of the transfer, 2 blastocysts can be selected if the woman has never given birth and if there is no scarring of the uterus.