The question of why the implanted embryo does not successfully implant has always been a question of great interest to fertility doctors, reproductive scientists and fertility-related practitioners. (1) Abnormal uterine structure, such as congenital malformation of the uterus (bicornuate uterus, septum in the uterine cavity), endometrial polyps, endometritis and fibroids in the uterine cavity. (2) Thin endometrium: Previous uterine scratching for abortion, irregular menstruation (persistent menstrual flow) or poor blood circulation can cause endometrium. (3) Persistent hydrocele in the uterine cavity: Oedema and incompetence of the fallopian tubes can cause hydrocele in the uterine cavity. This is because the water in the fallopian tubes is blocked at the umbrella (caudal end) of the fallopian tubes, causing the water in the tubes to flow towards the uterine cavity. Persistent fluid accumulation in the uterine cavity can cause embryo implantation failure. However, in patients with polycystic ovaries, they may occasionally experience fluid accumulation in the uterine cavity after receiving ovulation stimulation medication, but this does not cause the problem of failure of implantation. (4) Poor quality or insufficient amount of adhesion protein secreted by endometrial cells or too much or too little amount of certain cytokines in the uterine cavity can affect the success of embryo implantation. (5) It is indisputable that autoimmune disease or thrombosis is associated with habitual miscarriage, but it is inconclusive whether there is a correlation with repeated embryo implantation failure. (2) The embryo itself is defective when implanted in the uterus: (1) The chromosomal abnormalities of the couple will affect the development of the embryo, so that the embryo will stop growing at a certain stage. The wife has chromosomal abnormalities, like chromosomal balance alignment, chimeric chromosomes, reverse chromosomal translocation, chromosomal deletions, chromosomal breaks (especially if the break is in the central segment of the chromosome, the chances of repeat failure of IVF will be higher even in young women). In some patients with sparse spermatozoa and poor motility, their spermatozoa were subjected to fluorescent antibody hybridization to check for normal spermatozoa chromosome number, and it was found that the percentage of abnormal spermatozoa chromosome number was higher than that of those with normal spermatozoa and normal motility. Repeat IVF pregnancy failure cases were found to be as high as 60% of embryos with chromosomal abnormalities, so many embryos implanted in the uterus failed to implant because of chromosomal abnormalities in the embryos themselves. These chromosomally abnormal embryos sometimes appear to be of normal cell size, divide at a normal rate, and are good grade 1 or 2 embryos on the embryo classification scale. The older the recipient woman is, the greater the chance of abnormal embryos. (2) The zona pellucida, which surrounds the egg, is used to protect the egg and embryo before fertilization and before embryo hatching and implantation. If the embryo has been incubated in vitro for too long, the zona pellucida will become thicker and harder; older women will have thicker zona pellucida. Too thick or too hard zona pellucida may cause failure of embryo implantation. (3) Poor in vitro culture environment, including poor culture fluid, improper temperature or poor handling techniques, may damage the embryo and cause embryo implantation failure.