The first half of menstruation refers to the menstrual and proliferative phase; the second half of menstruation refers to the secretory phase. 1. Menstrual phase: The menstrual phase refers to days 1-4 of the onset of the menstrual cycle, during which the spongy functional layer of the endometrium is shed from the basal layer due to a decrease in the levels of progesterone and estrogen, and the onset of menstruation occurs. 2. Proliferative phase: It is the 5th-14th days of the menstrual cycle, including the 1st-4th days of the menstrual period belonging to the first half of the menstrual period. This phase is the follicular phase, in which the endometrium under the action of estrogen shows proliferative changes in the epithelium, glands, mesenchyme and blood vessels. 3. Secretion period: the 15th-28th days of menstruation, corresponding to the luteal phase of the ovary; this period of the endometrium will continue to thicken, the glands grow more curved, the phenomenon of secretion, and contains a wealth of nutrients, which is conducive to the development of fertilized ovum bedding. In the clinic, the above menstrual cycle is introduced with 28 days of menstrual cycle as an example; if there is any abnormality in the menstrual cycle, you need to go to the hospital for examination and treatment in time to avoid delaying your condition.