Today, in the vast majority of IVF cycles, multiple embryos are produced due to superovulation. To increase the live birth rate and reduce the rate of multiple pregnancies, 1-2 best embryos are usually selected for transfer. The extra embryos are frozen and preserved. Choosing good embryos will play a crucial role in improving the success rate of IVF, so how do you choose good embryos? The most common method of embryo selection is morphological evaluation, which screens for suitable embryos for transfer based on morphology. I. Embryo growth rate Embryo development is 16-20 hours after fertilization when the sperm and egg appear as two progenitor nuclei, the egg divides into two cells when it is 24-26 hours, four cells when it is 44-48 hours, 6-8 cells when it is 64-72 hours, and it becomes a blastocyst after 105-110 hours. Nowadays, the IVF treatment performed in Shanghai Sixth Hospital is usually performed on the third day after egg retrieval, when the embryos should be 6-8 cells, if the embryos do not reach this number or grow too fast, they are not very good. The morphology of the ovoid sphere in the embryo is whether each cell in the embryo is the same size, similar in shape and symmetrical to each other. The first choice should be embryos that are consistent in size and symmetrical in shape, and these are often referred to as quality embryos. Similarly it is also believed that one can determine if an embryo is of good quality by observing the morphology of both male and female primordial nuclei that appear in the egg 16-20 hours after fertilization. Third, the amount of fragmentation in the embryo Many IVF centers consider embryos with less than 5% of the total volume of fragmentation as the best, those with less than 30% as usable, and those with 50% or more as unusable for transfer. Some foreign fertility centers will extract the fragments from the embryos by microscopic manipulation and then transfer the embryos, believing that this will increase the implantation rate of the embryos. However, the consensus is to try to choose embryos with fewer fragments, which are more likely to be viable after the transfer. Fertility experts point out that the low success rate of IVF technology is due in large part to the fact that embryos produced by in vitro fertilization often have problems such as chromosomal abnormalities (including abnormalities in chromosome number or structure). The rate of chromosomal abnormalities in embryos obtained by assisted reproduction techniques reaches 40-60%. These chromosomally abnormal embryos, if implanted in the uterus, will not implant and develop properly, ultimately leading to failure of embryo implantation. How to screen for quality and healthy blastocysts? Pre-implantation chromosome screening (PGS): By taking blastocyst trophectoderm cells as biopsy material, the 23 chromosomes of the embryo are tested for copy number, and the results are used as reference to screen the best quality blastocysts for implantation, taking into account the morphology and chromosomes of the embryo, thus minimizing fetal abortion and miscarriage due to chromosomal abnormalities and ensuring the health of the fetus and the mother. Therefore implantation of healthy blastocysts will increase the rate of embryo implantation, and for those requiring twins will implant two embryos is also very helpful for success. This is why top-notch embryologist and five-day blastocyst culture techniques are critical, including the doctor’s ability to find the right window for transfer is important for IVF success.