Under normal circumstances, what is the normal thickness of the endometrium before menstruation? First of all, we need to find out what endometrium and the changes of endometrium so that we can understand clearly what is the normal value of endometrial thickness before menstruation. The endometrium is divided into a basal layer and a functional layer. The basal layer is not affected by ovarian hormonal changes during the menstrual cycle and does not shed during menstruation; the functional layer is affected by ovarian hormones showing cyclic changes and necrosis and shedding during menstruation. Therefore, the thickness of the endometrium is not a fixed value, it will vary cyclically with the menstrual cycle. Let’s take a menstrual cycle of 28 days as an example, the cyclic changes of endometrium can be divided into three phases: 1. Proliferative phase: the 5th-14th day of the menstrual cycle, which corresponds to the maturation stage of follicular development. (1) Early proliferative phase: 5th-7th day of menstrual cycle. The proliferation and repair of the endometrium starts during the menstrual period. The endothelium is thin at this stage, only l-2 mm, and the glandular epithelial cells are rectangular or low columnar in shape. The interstitium is denser and the cells are astral in shape. The small arteries in the mesenchyme are straight and have thin walls. (2) Mid-proliferative phase: 8th-10th day of the menstrual cycle. This period is characterized by marked interstitial edema; the number of glands increases, grows and is curved; the glandular epithelial cells are actively proliferating, the cells are columnar in shape, and there are signs of division. (3) Late proliferative phase: the 11th-14th day of menstrual cycle. At this stage, the endothelium is thickened to 3-5 mm, and the surface is uneven and slightly wavy. The epithelial cells are highly columnar, the glandular epithelium still continues to grow, the nuclear schizophrenia increases, and the glands are longer and form a curved shape. Mesenchymal cells are stellate, and combined with each other into a network; tissue edema is obvious, small arteries are slightly curved, the lumen is enlarged. 2.Secretory phase: After the formation of corpus luteum, under the action of progesterone, the endometrium shows secretory reaction. (1) Early secretory phase: the 15th-19th day of menstrual cycle. The endometrial glands are longer and the flexion is more obvious in this period. Small glycogen-containing vesicles, called subnuclear vacuoles, begin to appear under the nucleus of the glandular epithelial cells, which are the histological features of the early secretory phase. (2) Mid-secretory phase: The 20th-23rd day of menstrual cycle. The endothelium is thicker and more serrated than before. The apical membrane of the secretory epithelial cells in the gland is ruptured and the intracellular glycogen is discharged into the lumen of the gland called parietal pulp secretion. During this period, the plasma is highly edematous and lax, and the small spiral arteries are proliferated and coiled. (3) Late secretory phase: 24th-28th day of menstrual cycle. This period is the pre-menstrual period. The endometrium is thickened in a spongy manner. The endometrial glands open facing the official cavity, with secretions such as glycogen overflowing, the interstitium is more lax and edematous, and the interstitium under the surface epithelial cells differentiates into hypertrophic ecdysteroid cells. At this stage, the small spiral arteries grow rapidly beyond the thickness of the endometrium and are also more curved, and the lumen of the vessels is dilated. At the late stage of secretion, the thickness of the endometrium is about 5-6 mm. 3. Menstrual period: the 1st-4th day of the menstrual cycle. At this time, the estrogen and progesterone levels drop, which activates the synthesis of prostaglandins in the endometrium. Prostaglandins can stimulate the contraction of the myometrium and cause continuous spasm of the small spiral arteries in the functional layer of the endometrium, and the endometrial blood flow is reduced. The area of damaged ischemic necrotic tissue gradually expands. Tissue degeneration and necrosis increase the permeability of the vascular wall, resulting in rupture of the vessels leading to the formation of a hematoma at the base of the endometrium, which leads to necrosis and exfoliation of the tissue. The degenerated and necrotic endometrium is mixed with blood and discharged, forming menstrual blood. The thickness of the endometrium can reach 8-10 mm, and the menstrual cycle is mainly regulated by the hormones of the ovaries, if the hormones of the ovaries are disrupted, it will also cause menstrual disorders and corresponding morphological changes in the endometrium. The endometrial changes caused by various reasons are mainly manifested clinically as amenorrhea and “gongbao”. It is necessary to go to a specialized hospital for treatment in a timely manner.