I have been in contact with patients doing IVF basically every day for the past 2 years, and this question is often asked to me. Whenever I am asked, I don’t know how to answer because it is not a simple yes or no question, but also needs to be explained in detail. On the day of egg retrieval, the egg and sperm fertilize and unite, and 3 days later a cellular stage embryo is formed, which we call ’embryo’ for short. If the culture continues for 2 days, that is, 5 days after egg retrieval, the embryo becomes a blastocyst, and generally speaking, a good quality embryo can eventually form a blastocyst, and a poor quality one may not form a blastocyst, which means that during the 2 days of continued culture Therefore, we believe that for the same embryo, the blastocyst is more representative of its growth potential, and based on this, the success rate of blastocysts is currently higher than that of embryos for the same patient. Can we therefore give the answer that blastocysts are better than embryos for transfer, but of course not, because there are some special cases, for example, in the same patient, 6 embryos were formed on day 3, and it is prudent to select the 2 best ones for freezing and preservation first, and the remaining 4 continue to culture, and these 4 eventually formed blastocysts, then for this patient, we still prefer to transfer the 2 embryos frozen on day 3, why? Why? Because those 2 are the best ones in the same cohort and if they continue to culture at that time, there is every possibility that they will form even better blastocysts. Of course, blastocyst culture is something we do based on certain conditions, and it is not recommended for all patients at this time. Some patients have few eggs and few embryos, and blastocyst culture is risky, and there is a possibility of culture failure, in which case we still recommend embryo transfer. In other words, for any patient, blastocyst culture may be at risk of culture failure, although with successful culture, the pregnancy rate is also relatively increased. I am not sure if I have explained this clearly, but at least it will help more people to understand the difference between blastocyst and embryo, and I hope that it will be helpful for patients doing IVF to see it. What is blastocyst transfer In vitro fertilization – embryo transfer is a routine treatment in which two to three embryos are transferred on the third day, when the embryos are usually developed to 8-10 cells. If the embryos continue to be cultured until day 5-6, they will develop to the blastocyst stage. This is the period of embryo development that is referred to as blastocyst culture for IVF. Advantages of blastocyst transfer In practice, blastocyst culture requires high conditions and only good quality embryos can develop to the blastocyst stage, while poor quality embryos can be naturally screened out due to their own developmental abnormalities during the process of culturing to the blastocyst stage. This is also a process of elimination of the best and the worst. If the embryos have poor developmental potential, they may stop developing and stagnate at the day 3 stage. Therefore, it is not possible to measure the developmental potential of day 3 embryos (8-cell stage) with sufficient scientific accuracy, and it is difficult to predict the later developmental potential of embryos transferred during this period. Moreover, day 3 embryos arrive in the uterine cavity in an earlier state of development than those accepted by the endometrium during natural pregnancy. Therefore, embryos that are cultured in vitro beyond the 8-cell stage and become blastocysts become more viable embryos. Embryos that have been screened to develop into blastocysts are usually of better quality and have a higher transfer success rate. In addition, blastocyst culture allows the embryo to develop more synchronously with the female reproductive tract, more in line with the natural environment of reproductive physiology Reduces the influence of certain adverse factors between mother and fetus, and has a higher potential for implantation and pregnancy. In recent years, with the improvement of IVF laboratory technology, blastocyst in vitro culture system has been optimized and blastocyst rate has been gradually increased; at the same time, blastocyst freezing technology has also matured, thus making blastocyst transfer possible as a routine transfer technique. What is single blastocyst transfer and why single blastocyst transfer Single blastocyst transfer is the transfer of one blastocyst on the 5th day after egg retrieval. Some centers have now adopted the blastocyst stage (D5 or D6 day embryos) for embryo transfer. Two or more embryos are usually transferred at the time of embryo transfer to increase the pregnancy rate, which increases the chance of multiple pregnancies and increases the risk of pregnancy and delivery. There is a clear relationship between the number of pregnancies and adverse pregnancy outcomes, but most patients are not aware of the risks of multiple pregnancies, and some even undergo IVF to have twins. The incidence of maternal complications such as gestational heart disease, amniotic fluid embolism, preeclampsia, gestational diabetes mellitus and postpartum hemorrhage are significantly higher in multiple pregnancies (including twins), and the chance of hysterectomy and transfusion therapy is greatly increased. Some studies have confirmed that twins are eight times more likely to have low birth weight than a singleton; twins have a five times higher stillbirth rate and a seven times higher neonatal mortality rate than a singleton; and twin babies are four times more likely to have cerebral palsy than a singleton. The best solution to these problems is single blastocyst transfer, and single blastocyst transfer does not reduce the pregnancy rate. This shows that single blastocyst transfer can effectively reduce the rate of twin births while maintaining the pregnancy rate, reducing the risk to the fetus and the mother. What is selective single blastocyst transfer Currently, single blastocyst transfer is usually performed using the “selective single blastocyst transfer method”. This method is based on the patient’s age, history of infertility, and the availability of good quality embryos on the third day after egg retrieval. Why is selection necessary? This is because while blastocyst transfer has its advantages, it also has its disadvantages. Not all patients are suitable for single blastocyst transfer. Therefore, different individualized transfer protocols should be used for different patients.