The menstrual cycle starts on the first day of menstruation and ends on the day before the next menstrual period, and is divided into follicular phase, ovulation phase and luteal phase. A menstrual cycle in women with regular menstruation lasts about 21-35 days, with an average of 28 days. Based on a 28-day menstrual cycle, during the late luteal phase of the previous menstrual cycle and the early follicular phase of the current menstrual cycle (day 1-4 of menstruation), a group of sinus follicles (about 3-11) will enter the growth and development track together with the action of follicle stimulating hormone (FSH) in the ovary, while the other follicles will be occluded, a process called recruitment. Around day 5-7 of the menstrual cycle, the dominant follicle is selected and the follicle with the lowest FSH threshold, i.e. the one most sensitive to FSH, will develop into the dominant follicle first, while the other follicles will gradually atrophy due to insufficient FSH support. During the late follicular phase (day 11-14 of menstruation), when the dominant follicle matures, it will secrete high levels of estrogen, about 200-300 pg/mL. The high level of estrogen has a positive feedback effect on the hypothalamus, which releases large amounts of gonadotropin-releasing hormone (GnRH), which controls the secretion of FSH and LH by the pituitary gland, forming the LH peak, thus initiating ovulation. After ovulation, the follicle wall collapses and the surrounding smooth muscle contracts, forming the corpus luteum. The corpus luteum secretes estrogen and progesterone, which suppress pituitary and hypothalamic secretions and inhibit follicular development. If the ovum is not pregnant, the luteal phase lasts about 14 days and the corpus luteum atrophies, the concentration of estrogen and progesterone decreases and the negative feedback is lifted, thus starting the next menstrual cycle. Therefore, for women with regular menstruation, ovulation is about 14 days before the next menstrual cycle. Other methods to assist in determining ovulation are as follows 1. Basal body temperature BBT: The increase of progesterone after ovulation will stimulate the thermoregulatory center of the hypothalamus, so the basal body temperature in the luteal phase will be 0.3~0.5℃ higher than that in the follicular phase. However, the biphasic basal body temperature cannot predict the time of ovulation, but can only indicate that ovulation has already occurred. 2.Cervical mucus: During the ovulation period, influenced by the high level of estrogen, the secretion of cervical mucus will increase significantly and become very thin and transparent, with a drawing degree of up to 10cm or more, and lambdoid crystals can be observed under the microscope after smearing. 3.Urine LH test paper: Before ovulation, LH will rapidly form LH wave peak, LH peak from the start lasts about 48h, monitoring urine LH test paper measured double bar indicates imminent ovulation, but the urine LH test paper is not accurate in patients with improper use, irregular menstruation, polycystic ovary syndrome. 4.Ultrasound ovulation monitoring: Patients who have irregular menstruation, have not detected ovulation by self-monitoring or are still infertile during ovulation, can have their ovulation monitored by ultrasound at the fertility center and have intercourse under the guidance of the doctor to increase the chance of pregnancy. If ovulation is not detected after 3-6 cycles of monitoring, further tests are recommended to find the cause of infertility.