Some of you may know that on the 3rd day of egg retrieval, if you have enough embryos, you need to sign an informed consent form about blastocyst raising. Some people “know a little bit” about “blastocysts are better”, while some people don’t understand why they need to raise blastocysts at all, especially if they find out on day 5 that they got few or no blastocysts, and of course, some people don’t have any idea about it or even don’t remember it. So, what exactly is blastocyst rearing? What is the significance of blastocyst rearing? Is it good or bad? This article will answer these questions for you. The process of embryonic development To answer these, first we need to understand the process of embryonic development and what exactly is going on with blastocysts. Cells proliferate by dividing in two, and after the egg is fertilized, the process of successive one-to-two, two-to-four, and four-to-eight divisions begins. The day of egg retrieval is designated as day 0, and the subsequent days of embryonic development are as follows: As can be seen, the egg is continuously changing after fertilization, with each day having a very different morphology. As described above, the day 3 embryos are called cleaved embryos, the day 5 embryos are called blastocysts, the day of transfer is called fresh embryos, and the day of freezing is called frozen embryos. The first clinical transfer or freeze is usually on day 3 or day 5, i.e., the cleaved or blastocyst embryo. The day 3 cleaved embryo can continue to be cultured to obtain a blastocyst, a process, which is called blastocyst raising. So, why do we need to raise the blastocyst? What are the benefits of blastocyst culture? Benefits of blastocyst rearing The average live birth rate for transferring an oogenesis embryo is reported to be around 30-35%, while the live birth rate for a blastocyst can reach 50-60%. The advantages of blastocyst culture and blastocyst transfer overlap to a large extent because blastocyst rearing means more likely blastocyst transfer: 1) blastocyst transfer is more synchronized with the endometrium on day 5 after egg collection and has a higher implantation rate; 2) blastocysts have better developmental potential; 3) blastocyst transfer reduces the rejection of blastocysts by the uterus and has a lower chance of being expelled; 4) blastocyst culture is a self-screening process that allows for the screening of higher quality embryos. 4. blastocyst culture is a self-screening process, which can screen out better quality embryos and reduce the waste caused by bad embryo transfer, especially when there are many embryos; 5. blastocysts are more resistant to freezing than oocytes and have strong repair ability after freezing damage; 6. blastocysts can be more accurately tested for PGS/PGD (genetic testing) and time-lapse technology, which further helps to screen out quality embryos; 7. blastocyst transfer has a significantly lower probability of ectopic pregnancy than oocyte transfer. The probability of ectopic pregnancy in blastocyst transfer is significantly lower than that in oogenesis transfer; 8, blastocyst transfer usually involves only one embryo, which reduces the incidence of multiple pregnancies. Disadvantages of blastocyst transfer Have you been fascinated by the advantages of blastocyst transfer and feel that it is necessary to raise blastocyst? In clinical practice, however, blastocyst culture is not recommended for all embryos. This is because, despite the numerous advantages of blastocyst culture, the current state of the art does not accurately predict which oogenesis embryo will go on to develop into a blastocyst. Blastocyst culture has the following disadvantages: 1. If all embryos are used to raise blastocysts, by day 5 after egg retrieval, no blastocysts are formed and no embryos are available; although the probability of this happening is small, it can still happen, especially if it is prone to happen in a certain special group of people (to be mentioned later), for this group of people, perhaps direct transfer of cleaved embryos is a better choice instead; 2. Although single Although single blastocyst transfer can significantly reduce the rate of multiple births without reducing the pregnancy rate, the incidence of monozygotic twins in blastocyst transfer is significantly higher; 3. Blastocyst culture means that embryos stay longer in vitro, and the effect of culture fluid on embryos needs longer time and more studies to confirm, and some studies found that blastocyst transfer may increase fetal birth weight and disproportionate sex ratio. Blastocyst culture criteria and strategies Now that we know that blastocyst culture has both advantages and disadvantages and that blastocyst raising is not suitable for everyone, should we raise blastocysts or not? When should blastocyst culture be performed? In this regard, in 2013, the American Society for Reproductive Medicine (ASRM) proposed the following strategies for blastocyst culture: 1. For patients with relatively young women, good ovarian reserve, and many eggs (>8 mature eggs), blastocyst culture can improve the live birth rate, and single blastocyst transfer is recommended; 2. 3. For patients with “poor prognosis” of IVF outcome (advanced age, low ovarian reserve, low number of eggs obtained), blastocyst transfer does not increase the live birth rate and blastocyst culture is not recommended. As with most centers, our center generally chooses to transfer cleavage-stage embryos (fresh embryos) first and then cultured blastocysts from the remaining embryos, as appropriate, depending on the patient’s circumstances. This strategy allows for further screening of embryos through blastoculture while ensuring the availability of embryos for transfer, saving the time and financial costs associated with multiple freezing of oogenic embryos and multiple transfers, and better protecting the interests of patients.