1. Follicular phase (days 5-14 of menstrual cycle): From day 5 of menstruation, a new batch of follicles (10-15 candidates) starts to grow in the ovary and preferentially selects the main follicle, which can be seen by ultrasound on about day 9. Estrogen levels rise significantly at this stage. The endometrium starts to grow from 1-2mm on day 6 of menstruation and should be 8-10mm by the time of ovulation. 2. Ovulation (one of the 14th, 15th and 16th days of the menstrual cycle): the egg breaks the sac from the dominant follicle and estrogen rises to its peak. Estrogen decreases briefly when the egg is expelled (the very reason for the small amount of bleeding during ovulation). The endometrium should be 9-12 mm during ovulation. 3. Luteal phase (days 15-29 of the menstrual cycle): After ovulation, the follicles form the corpus luteum, which secretes large amounts of progesterone and estrogen. Progesterone rises to its peak on day 22-23, when progesterone levels are most reliably measured and luteal function is assessed. The endometrium continues to thicken to 15-19mm before menstruation. 4. Menstrual period (1-5 days of the menstrual cycle): the corpus luteum atrophies, estrogen and progesterone decline, and the endometrium withdraws and bleeds. Because of this, some detailed examinations related to gynecological endocrinology are very time sensitive: “sex hormone 6” requires blood sampling on the 3rd day of menstruation; breast examination is best done 1 week after menstruation; vaginal ultrasound monitoring of ovulation should be done from the 9th day of menstruation, and then once before and once after ovulation (while measuring endometrial thickness). Lastly, what is meant by menopause? Menopause generally refers to the age of about 50, and menstruation stops after one year, even if you have entered menopause. Diagnostic indicators are as follows: follicle stimulating hormone 40U/L, estradiol low to below 39nmol/ml, endometrium below 5mm.