People who are doing IVF often ask the question: Why do I not have as many embryos in the end, even though I have a good number of eggs? What is the reason for this? The process of forming a new human life is very sophisticated and complex. An egg from the mother and a sperm from the father combine to form a fertilized egg, which then undergoes a series of divisions, differentiation, development and eventually becomes a baby. But not every egg has the privilege of developing into a new life. Compared to other species, the utilization rate of human eggs is actually very low, why? Let’s take a look below to find out. To have a mature, healthy egg The most important thing for a successful pregnancy is to have a mature, healthy egg. Not only because half of the genetic material in an embryo has to come from the egg, but also because the egg provides the material conditions for early embryonic development. Therefore, the quality of the egg is very critical. Generally speaking, the quality of eggs decreases with age. Some studies have shown that the quality of eggs begins to decline significantly in women over the age of 35, so it is best for women to have children before the age of 35. As women age, the number of eggs becomes less and less, and at the same time, the quality declines rapidly, so fertility declines, a trend that cannot be reversed by current medical science. In addition to age, egg quality is also related to genetics and environment. In addition to the rare genetic influence, most women’s egg quality is also influenced by the environment. We can try to improve egg quality by avoiding adverse environmental influences, such as trying not to come into contact with reproductive toxic chemical products such as pesticides, paints, heavy metals, etc. Good lifestyle and diet are also conducive to producing healthy eggs. Only mature eggs can be fertilized Not all eggs obtained during IVF are mature, the maturity rate is usually about 80%, only mature eggs can be fertilized, the fertilization rate of conventional IVF is usually about 70%, after fertilization not all eggs can be completed to develop into embryos, the egg cleavage rate is usually more than 95%. After fertilization, 24-30 hours after fertilization, the fertilized egg begins to divide, with one cell splitting into two and the two newly formed daughter cells being essentially equal in genetic material and cytoplasm. Each cell of the embryo has a special name called oocyte, and the normal rate of division in an embryo is 4 oocytes observed on day 2 after egg retrieval, 8 oocytes observed on day 3, a mulberry embryo of about 100 cells formed on day 4, and a blastocyst with an inner cell mass in the middle and an outer trophoblast layer on day 5. Recent studies have shown that the developmental potential of embryos decreases if their early division rate is slower than normal. Therefore, a 4-cell embryo with uniform cell size on day 2 is a good quality embryo; an 8-cell embryo with uniform cell size and little debris on day 3 is a good quality embryo; a good quality embryo on day 4 is a mulberry embryo; an embryo on day 5 should develop into a blastocyst; and a blastocyst hatches from the zona pellucida on day 6. Early embryo division that is too fast or too slow suggests some abnormality in the embryo. As you can see above, not as many eggs as there are embryos will eventually result in as many embryos as there are eggs. In some cases, because of poor egg quality or sperm quality problems, the fertilization rate, egg cleavage rate, and quality embryo rate are significantly reduced, and sometimes there are no good embryos available for transfer. There are also patients who have good quality eggs per egg, so they are lucky to end up with the same number of good quality embryos, but overall, the rate of human embryos formed per egg available is not high. Analysis of data from our center’s previous 6000 cycles shows that the egg maturation rate was 84.77%, the fertilization rate was 74.5%, the oogenesis rate was 95.0%, and the stalling rate per oocyte protoplast was 3.20%. The probability of forming day 3 usable embryos per oocyte cleavage was 73.98%, and the utilization rate of day 3 embryos per egg formed was 45.98%; the utilization rate of blastocyst cultured embryos was 53.13%, thus calculating a blastocyst utilization rate of 24.43% per egg. In layman’s terms, this means that for every 10 eggs, 4-5 good usable D3 embryos or 2-3 good usable blastocysts can be formed. Our center’s figures are above the international average. According to international reports, in the first years of IVF technology, 6.8-11 eggs were needed per pregnancy and an average of 15 eggs per live birth. With the development of IVF technology and clinical experience, the clinical pregnancy rate has been increasing, however, the egg utilization rate has not improved significantly. With the use of ovarian stimulation, an average of 9-12 eggs can be obtained per egg retrieval cycle, but recent studies have shown that the live birth rate per egg is only 6.8% per mature egg even in the highest donor cycles, indicating that egg utilization is still low after more than 30 years of development, with more than 90% of eggs ultimately do not develop into live births.