Blastocyst, an embryo consisting of an inner cell mass, a blastocyst cavity, and a trophoblastic ectoderm, is formed when the fertilized egg is 5-6 days old. The blastocyst is the ultimate stage of embryonic culture in vitro and the stage at which the human embryo implants into the mother. Blastocyst transfer results in a higher implantation rate because blastocysts are more mature and better suited for growth in the uterine cavity than oogenic embryos. Many patients want to have good quality blastocysts, so let’s see what factors are involved in the formation of blastocysts. Environmental Factors Blastocyst culture requires a strict culture environment. Embryos are removed from the protection of the fallopian tube and uterus and are susceptible to damage from external environmental changes, so the incubator and culture fluid mimic the body’s protection of the embryo by maintaining the pH, temperature and relative humidity for embryo growth in vitro. With the advancement of embryo culture fluid and culture technology, a relatively stable and mature process has been formed for blastocyst culture method. In the IVF workstation with 100 levels of laminar flow purification (the highest purification level), all environmental indicators are monitored in real time, and such a stable and safe culture environment is the basis for embryos to be able to mature in vitro. Embryonic factors The culture of blastocysts is the process of further screening of embryos. More than half of the embryos developed to the third day (oogenesis) are chromosomally abnormal embryos. And after 5-6 days of in vitro culture, a part of the chromosomally abnormal embryos stalled their development or developed morphological abnormalities, and only a small number of embryos with good developmental potential formed beautiful blastocysts. In most laboratories in China, the morphologically best oogenesis embryos are preferentially frozen on the third day, and blastocysts are the embryos that win by further culturing the remaining embryos and eliminating the weak and obviously defective partners. So the formation of blastocyst is the result of the embryo’s own merit. Female age Female age is one of the independent factors affecting embryo quality, and its importance is well understood, so I will not dwell on it here. Sperm quality has been shown to be an important factor influencing the rate of quality embryo formation, including sperm motility, morphology and DNA fragmentation index (DFI), after excluding differences in age, fertilization method, and ovulation protocol. Patients with low sperm motility, large-headed sperm, and DFI >30% have a significantly lower rate of high-quality blastocyst formation. Progesterone levels Patients with high HCG day blood progesterone often forgo fresh cycle transfer because of the effect of progesterone on the endometrium. In contrast, it has been shown that patients with HCG day P >1.49 ng/ml have a low rate of high-quality embryo formation. The reason for this result is unclear; one of the possible reasons is that patients with high progesterone tend to have a longer duration of ovulation promotion, and the lower blastocyst formation rate may be related to a longer duration of ovulation promotion in order to obtain more mature eggs. Of course, the culture of blastocysts is not a hundred percent beneficial. The environment in which the embryo grows in the mother’s body cannot be fully simulated by the laboratory, just as formula cannot be fully equated with breast milk. The longer embryos are cultured in vitro, the more they suffer from environmental influences, which is why embryo culture laboratories preferentially transfer or freeze third-day oogenesis embryos. However, blastocyst culture is again one of the scientifically valid methods in order to verify the developmental potential of embryos and to select good quality embryos. With the development of embryology, blastocyst culture will also be studied more intensively. Therefore, we need to take a dialectical view of blastocyst culture, and the doctor will make the most reasonable choice of transferring, freezing or continuing the culture of embryos based on the patient’s actual situation and weighing the many pros and cons.