The endometrium is divided into a functional layer and a basal layer. The thickness of the endometrium of the functional layer varies in different periods of the menstrual cycle and will change periodically with the change of estrogen and progesterone during the menstrual cycle. A menstrual cycle lasts 28 days and is divided into three periods: proliferative phase, secretory phase and menstrual phase. The thickness of the endometrium at different periods is: 1. Proliferative phase: The proliferative phase is on days 5-14 of the menstrual cycle, when the endometrium is relatively thin and may grow to about 3-5mm overall. The proliferative phase can be divided into three periods: early proliferative phase is day 5-7 of the menstrual cycle, when the lining is thin, only about 1-2mm; middle proliferative phase is day 8-10 of the menstrual cycle; late proliferative phase is day 11-14 of the menstrual cycle, when the lining can grow to 3-5mm; 2. secretion phase: which is day 15-28 of the menstrual cycle, when the lining grows thicker. The whole secretion phase is also divided into three periods: the early secretion phase is day 15-19 of the menstrual cycle, the middle secretion phase is day 20-23, and the late secretion phase is day 24-28. During the secretory phase, the endometrium grows to about 10mm; 3. Menstrual phase: refers to the 1st-4th days of the menstrual cycle, when the change of hormone level in the body is relatively large, especially the rapid decline of estrogen and progesterone, resulting in the shedding of the endometrium, and the thickness of the endometrium is about 0.5mm. In general, the thickness of the endometrium changes with the menstrual cycle, from thin to thick and then to thin again.