1. What is a single blastocyst transfer? As the name suggests, single blastocyst transfer is the transfer of one blastocyst on the 5th day after egg retrieval. 2. What are the benefits of single blastocyst transfer? The true success of assisted human reproductive technology treatment is to obtain a single full-term live birth. Traditional IVF treatment often involves the transfer of 2-3 blastocysts depending on age, which increases the clinical pregnancy rate while increasing the risk of multiple pregnancies and ovarian hyperstimulation syndrome. Multiple pregnancies are prone to adverse pregnancy outcomes such as miscarriage, preterm delivery, intrauterine growth retardation, and gestational hypertension syndrome, and are considered serious complications of assisted reproductive technology rather than successful assisted conception outcomes. Multiple pregnancies are associated with a significant increase in perinatal morbidity, mortality, miscarriage, preterm delivery, and cesarean section rates. 2/3 of twin pregnancies result in low gestational age, and the probability of having at least one child with birth defects in twin, triplet, and quadruplet pregnancies is 7.4%, 21.6%, and 50%. The incidence of cerebral palsy is 6 and 10 times higher in twin and triplet pregnancies, respectively, than in singleton pregnancies. United Nations studies have shown that 56% of the cost of IVF is associated with multiple pregnancies, with the perinatal maternal and infant costs of IVF being £3313 for a singleton pregnancy, compared to £9122 for twins and £32354 for triplets, with the perinatal cost of a triplet being almost 10 times that of a singleton pregnancy. Reproductive experts have been seeking a reliable solution to this problem. Currently, there are two clinically effective ways to reduce the number of multiple pregnancies: either by performing reduction or by reducing the number of embryos transferred. The former is a remedial tool, while the latter is an active preventive tool. 3. Why choose a single blastocyst instead of a single oogenic embryo? Studies have confirmed that the clinical pregnancy rate of transferring a single blastocyst is significantly higher than that of transferring a single oogenesis embryo. This suggests that blastocyst may be the ideal period to achieve single embryo transfer. The blastocyst is an important developmental stage after the oogenesis embryo, morphologically undergoing changes of cell fusion, emergence of blastocyst cavity, and expansion of the blastocyst cavity. During this process, the development of poor developmental potential and chromosomal abnormalities will be stalled, and good quality oogenesis embryos will develop to the blastocyst stage. 4. Is everyone suitable for single blastocyst transfer? At present, only about 50% of good quality embryos at the cleavage stage can develop to blastocyst, and if blastocyst culture is performed completely, 20% to 40% of patients will cancel the transfer because no embryos develop to blastocyst. Therefore, blastocyst culture and transfer are not recommended for patients with advanced age, poor embryo quality, and previous IVF failure. Patients beyond that also make decisions based on the number and quality of embryos on the third day. This is called selective embryo transfer. 5. What is the outlook for selective single blastocyst transfer? Many studies have found that blastocyst transfer has a higher implantation rate and that selective single blastocyst transfer both increases cumulative pregnancy rates and does not decrease pregnancy rates per transfer cycle, and reduces multiple pregnancy rates and the risk of ovarian hyperstimulation. However, many physicians and patients are still concerned about the reduced success rate of single blastocyst transfer, and some patients pursue twin pregnancies, which makes the implementation of single blastocyst transfer more difficult. It is believed that with the development of science and technology and the change of public opinion, selective single blastocyst transfer can gradually be accepted as a safe and practical method to improve the clinical outcome of infertile patients.