As IVF technology matures, pregnancy rates are becoming higher and higher, currently around 50% in most centers, with some higher in individual centers, so the twin rate remains high, in some centers the twin rate is around 20-30% or even higher. In the beginning of IVF at each center, the focus was undoubtedly on pregnancy rates, but as pregnancy rates have increased and stabilized, reducing the twin rate has become a pressing issue in the field of reproduction. Most patients may ask, “What’s wrong with twins? I can understand that, because patients are not professional fertility or obstetrics and gynecology doctors, and they may think that twins are the envy of everyone around them, but what they don’t know is the risks involved in twin pregnancies. Compared to singleton pregnancies, twin pregnancies are associated with a greater likelihood of hyperemesis during early pregnancy and a significantly higher risk of hypertensive disorders of pregnancy, miscarriage, and preterm delivery in the middle to late stages. In patients with severe gestational hypertension, proteinuria, eclampsia and other symptoms can occur, even endangering the mother’s life, and some patients need to be induced, and the mother has the risk of hemorrhage after delivery; twin pregnancy is a heavy burden for the mother, and some patients with low height, or uterine malformation, or scarred uterus who have had a cesarean section or myomectomy, etc. have a higher rate of miscarriage in mid pregnancy. The majority of twin pregnancies are delivered at less than 36 weeks, or even 32 weeks or earlier, so these premature babies have extremely low weight, and it is not uncommon to see newborns weighing 3 or 4 pounds, or even lower weight. In addition to low weight, these newborns may have congenital underdevelopment of many organs and need to be hospitalized in the neonatal unit once they are born, which, in addition to the health concerns of the newborns, puts many families in a difficult situation due to the huge costs. There are also newborns with congenital developmental deficiencies, such as congenital heart disease, that will require surgery in the next 3-5 years. Currently in IVF treatment, most patients still transfer 2 embryos or blastocysts, however, selective single embryo or single blastocyst transfer has become a development in the reproductive world. Selective single embryo transfer means that out of all the embryos or blastocysts, the best one is selected for transfer, which can ensure a certain pregnancy rate and also reduce the risk of twin pregnancies. For example, in our center, for patients with ideal conditions in terms of age, uterus, endometrium and embryos, we perform blastocyst culture and finally select the best quality for transfer. Therefore, we believe that the pregnancy rate is related to the quality of the transferred embryos and not to the number of transfers. Of course, at present, our center still focuses on transferring 2 embryos, and is gradually carrying out selective single blastocyst transfer. Single embryo transfer is not blindly recommended for all patients, but at present, for patients with ideal conditions, including those who have 2 transplants with the possibility of twin pregnancies, or such a part of patients who cannot tolerate twin pregnancies due to height, weight, or uterine abnormalities, we recommend selective single embryo (single blastocyst ) transfer to ensure their pregnancy rate and to reduce the risks during pregnancy. Therefore, before recommending single embryo (blastocyst) transfer, we perform a comprehensive and thorough evaluation of the patient and our goal is to have a singleton, full-term, and healthy baby. For patients with slightly less favorable conditions, such as advanced age, poor ovarian reserve or conditions such as fibroids or adenomyosis, these unfavorable factors can affect the embryo implantation rate, so in this case, 2 embryos will still be transferred to increase their chances of pregnancy. In conclusion, selective single embryo (blastocyst) transfer is a future trend that requires adequate evaluation of the patient beforehand and is not recommended to be performed blindly in patients with less than ideal conditions.