Embryos in culture are generally divided into those at the oogenesis stage, which are day 3 embryos, and those at the blastocyst stage, which are cultured up to day 5-6. Blastocyst culture involves culturing embryos in a specific culture system up to day 5-6. Under physiological conditions, the embryo will not enter the uterine cavity until it reaches the blastocyst stage. Extending the culture of the embryo outside the body to the blastocyst stage for 5-6 days and then placing it in the uterus not only allows for the selection of embryos with higher developmental potential, but also better physiological conditions, which can increase the embryo implantation rate and reduce the rate of multiple pregnancies by selecting high-quality single blastocysts for transfer. Blastocysts are generally divided into stages 1-6, and the blastocyst scores of stages 3-6 can be divided into three levels as A, B and C. If they are 4AA, 5AA are the best quality embryos, 4AB, 4BA, 4BB, 3AA, 3AB or 3BA, 3BB are also quality embryos, and if they are CC level embryos, they are poorer embryos. Those who have 3 or more oogenic embryos can choose to continue further culture to blastocysts. Young patients, those who respond well to ovulation promotion, those who obtain more than 10 eggs, facilitate the screening of the highest quality embryos, i.e., they can culture blastocysts, or those who have failed repeated IVF, especially those who have transferred high quality and day 3 oogenic embryos and are still not pregnant, or those who are ready to do preimplantation genetics of the diagnostic treatment cycle, is also an option to do blastocyst culture. The success rate of transferring blastocysts for culture is higher than that of day 3 embryos because they have undergone further screening. The international trend now is also to perform single blastocyst transfer, and the chance of a good quality embryo forming a blastocyst is about 60%, which means that not all embryos can be cultured in vitro to form a blastocyst. For patients with a low number of embryos, it is possible that no blastocysts will form and the final choice is up to the patient to make and your doctor will give you the best advice.