The blastocyst is the final stage of embryonic in vitro culture and is usually formed on day 5-7 after fertilization of the egg. In its natural state, the human embryo implants in the form of a blastocyst into the mother. Blastocyst transfer is performed to obtain a high rate of embryo implantation. Therefore, blastocyst culture and blastocyst transfer is always an option of interest to both doctors and patients. So, is blastocyst culture really beneficial but not harmful? To answer this question, it is necessary to understand the differences between embryos cultured in vitro and those developed in vivo in their natural state. Some people may think that the modern medical science and embryo culture labs are mysterious places with lots of precision instruments and are one of the pinnacles of human technology. But in fact, in the natural state, the mother provides the embryo with no light, constant temperature, constant partial pressure of gases (the most appropriate partial pressure of carbon dioxide, oxygen and nitrogen for the embryo’s needs), the most rationed nutrients and the essential cytokines (all of which are dynamically renewed and without excessive accumulation of metabolic waste). These conditions are either only roughly simulated or not at all possible in the laboratory. It is clear that the embryo laboratory provides an environment in which the embryo can grow that is “hot and cold” compared to the environment provided by the mother. It is indisputable that the overall developmental potential of embryos cultured in vitro becomes more and more severely compromised as the culture time increases. Chromosomal studies of in vitro cultured embryos have shown that the longer the in vitro culture time, the greater the likelihood of chromosomal abnormalities in the embryo. Having said that, one might question: while the natural state is certainly superior to the laboratory environment, do not forget that the whole process of blastocyst formation in the natural state occurs in the fallopian tube, not in the uterine cavity! As we all know, the internal environment of the uterine cavity is very different from that of the fallopian tubes. The normal environment of the fallopian tube is certainly better than the laboratory for blastocyst culture, but who can be sure that the uterine cavity is still more suitable for “blastocyst culture” than the laboratory? There is really no precise study to compare with such a question. However, there are a few facts worth noting: 1) there are many patients with bilateral tubectomies who have transferred cellular stage embryos (embryonic form before blastocyst formation) and had successful pregnancies, which shows that the uterine cavity is capable of “alone” completing the “blastocyst culture” process; 2) the uterine cavity and the fallopian tube are more suitable for “blastocyst culture” than the laboratory. There is little difference between the uterine cavity and the fallopian tube, except for the differences in cytokine and nutrient composition, and in other aspects listed in the previous section. The uterine cavity is hardly different from the oviduct, except for the differences in cytokine and nutrient composition, which are listed above. With these observations in mind, it is easy to conclude that prolonged in vitro culture (blastocyst culture) is somehow “crippling” to the embryo. But things cannot be absolute. Is blastocyst culture useless? No! Until the embryo forms a blastocyst, embryologists can only look at the embryo and speculate about its “texture” and its developmental potential based on various morphological indicators (fragmentation rate, number of cleavage balls, etc.). Unfortunately, such morphological observations do not always provide an accurate indication of the developmental potential of each embryo (even in terms of the possibility of blastocyst formation). Therefore, the most reliable (although also somewhat silly) method is to culture them together to blastocysts in vitro and speak with facts. Also, since the microenvironment of the uterine cavity (and/or fallopian tubes) may be problematic in some patients due to pathogenic factors, it is not hard to imagine that it would be better to keep the embryos in the lab for blastocyst culture. In summary, my personal opinion: blastocyst culture, at least in most cases, is not the preferred embryo processing option. However, blastocyst culture is not useless to verify the developmental potential of the embryo or to circumvent the undesirable intra-maternal environment.