The LH peak during mid-menstrual period produces important structural and functional changes in the developing follicle, probably by inhibiting the oocyte maturation inhibitory factor and stimulating the resumption of meiosis to achieve final maturation, which includes maturation of the oocyte nucleus, cytoplasm and zona pellucida, the appearance of clusters of cells surrounding the oocyte mound, and granulosa cell luteinization and corpus luteum formation, all of which are important for subsequent fertilization and pregnancy. The timing of HCG injection is based on: 1. the size of the follicles, 1-2 dominant follicles with an average diameter of 1-18 mm. 2. if there are more than 10 developing follicles and 3 follicles with a diameter of 14 mm. 3. If the follicles are more than 10 and 3 follicles are 14mm in diameter, the cervical hyaline mucus is abundant and the pupillary phenomenon is very obvious. 4, The length of the patient’s past menstrual cycle can also be used as a reference. 5. The average E2 level of each large follicle is at least 1110 pmol/ml. HMG injection should be stopped and HCG injection should be given, and the eggs should mature in 34-36 hours (most of them reach mid-division II). Obtaining sufficient retrieved eggs depends on: (1) The superovulation protocol must enable follicle development to reach preovulation, where not only does granulosa cell aromatase activity increase, but also cell surface LH receptor production increases. If the follicle is immature without sufficient LH receptors, HCG cannot initiate the final maturation process. (2) The timing of HCG injection must be determined as appropriate. If HCG is injected too early, the mound will be small and tightly attached to the follicle wall, which makes aspiration not easy and few eggs will be recovered, and the recovered eggs will be immature, resulting in low fertilization rate and low pregnancy rate; if HCG is injected too late, the follicles will be overripe before ovulation, which is also detrimental to later development. Due to the continuation of HMG injection, E2 levels are rising, the occurrence of endogenous LH peaks increases, and premature luteinization is detrimental to egg fertilization. Differences in the dose of HCG used exist due to varying individual sensitivity, and these differences are related to the amount of FSH/HMG used on the production of LH receptors in the preovulatory follicles and their function. We routinely use HCG 10000Iu in numerous developing preovulatory follicles to produce adequate initiation, but there are animal studies that demonstrate that the use of high doses of HCG will -reduce pregnancy rates due to increased E2 levels and toxicity to the embryo. It is worth considering whether the dose of HCG used is individualized.