Artificial insemination (AI) is the non-coital, artificial injection of washed and treated male semen into the female reproductive tract in order to naturally fertilize the egg and sperm for the purpose of pregnancy. Since artificial insemination is the natural fertilization of sperm and egg inside the body, it is extremely important to choose the right time to do it. In principle, it is most appropriate to choose the time of ovulation for IUI, but it is difficult to capture the moment of ovulation. The time limit for sperm fertilization is about 48~72h, and the time limit for egg fertilization is about 12~24h. Therefore, the success rate of artificial insemination is higher when it is performed from 48 hours before to 12 hours after ovulation, and our center will determine the timing of artificial insemination according to the size of follicle, the blood or urine LH value, and the blood E2 value, and we usually choose to perform artificial insemination before ovulation. Timing of IUI 1. Natural cycle: For patients with regular menstrual cycle, ovulation usually occurs about 14 days before the next menstrual period, according to the patient’s menstrual cycle, you can choose the time to start monitoring the follicles, usually around the 8th to 10th day of menstruation, and at the same time according to the size of follicles to monitor the blood or urinary LH value, according to the LH peak combined with the size of follicles to decide the timing of IUI. The timing of IUI is determined based on the peak LH and the size of the follicle. Ovulation usually occurs 34-36h after the peak of blood LH, so generally choose the day after the peak of artificial insemination, monitoring until the follicle rupture. 2.Ovulation promotion cycle: Ovulation promotion cycle is suitable for patients with irregular menstrual cycle, ovulation disorder and small follicle ovulation, which can improve the success rate of artificial insemination. Commonly used ovulation promoting drugs are clomiphene, letrozole, gonadotropin, etc. Oral clomiphene is preferred, if the effect is not good, it can be switched to a small dose of FSH/HMG. In the ovulation promoting cycle, when the dominant follicle is ≥14mm, blood or urine LH is monitored, and when the dominant follicle reaches more than 18mm, it can be injected with HCG10,000IU to induce ovulation, and IUI will be carried out the next day of the HCG injection. Our center usually carries out artificial insemination on the next day after HCG injection and monitors the follicle until it ruptures. There is no definite conclusion on the number of IUIs per cycle. Some literature shows that for ovulation induction cycles, one or two IUIs do not make a difference in the pregnancy rate, therefore, monitoring should be strengthened to predict ovulation by combining the body temperature, ultrasound, cervical mucus, blood/urine LH and blood E2 levels, and accurate IUIs should be performed in order to increase the pregnancy rate.