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 shed during menstruation. Therefore, the normal thickness of the endometrium is not a fixed value, it will vary cyclically with the menstrual cycle. Transabdominal ultrasound differs from transvaginal ultrasound in that the measurement is generally the superimposed thickness of the two anterior and posterior wall endometrium layers, and usually only the more echogenic functional layer is measured (sometimes the basal layer is also included). The normal cyclic changes in endometrial thickness can be divided into three phases: 1. Proliferative phase: The 5th-14th day of the menstrual cycle, which corresponds to the mature stage of follicular development. The endometrial glands and interstitial cells are proliferating under the action of estrogen during the follicular phase. The proliferative phase is further divided into 3 phases: early, middle and late. (1) Early proliferative phase: the 5th-7th day of menstrual cycle. The proliferation and repair of the endometrium begins during menstruation. At this stage, the endothelium is thin, only 1 to 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 interstitium are straight and thin-walled. The ultrasound measurement is probably only 4-6 mm. (2) Mid-proliferative phase: 8th-10th day of the menstrual cycle. This period is characterized by marked interstitial edema; increased number of glands, growth and curved shape; active proliferation of glandular epithelial cells with columnar cells and splitting signs. The endometrium starts to grow in the middle stage of hyperplasia, and the thickness of the endometrium gradually becomes thicker and grows to 8-10 mm. the ultrasound may also be seen in the range of 8-10 mm. (3) Late proliferative phase: the 11th-14th day of the menstrual cycle. At this stage, the normal thickness of the endometrium thickens to 3-5mm, and the surface is uneven and slightly wavy. The epithelial cells are highly columnar, the glandular epithelium still continues to grow with increased nuclear schizophrenia, and the glands are longer and form a curved shape. The mesenchymal cells were stellate and combined with each other to form a network; tissue edema was obvious, small arteries were slightly curved, and the lumen was enlarged. In the late stage of hyperplasia, the endometrium may also be in the range of 9 to 10 mm as seen by ultrasound. 2, the secretion phase: after the formation of the corpus luteum, under the action of progesterone, the endometrium is secretory response. The secretory phase is also divided into 3 phases: early, middle and late. (1) Early secretory phase: 15th-19th of menstrual cycle. The endometrial glands are longer and the flexure is more obvious in this period. Small glycogen-containing vesicles, called subnuclear vacuoles, begin to appear under the nucleus of glandular epithelial cells, which are 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. 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 towards the official cavity, with secretions such as glycogen overflowing, the interstitium is more lax and edematous, and the interstitium under the surface epithelium 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. In late secretion, the normal thickness of the endometrium is about 5-6 mm. in late secretion, the endometrium may be in the range of 10-12 mm as seen by ultrasound. 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 endothelial 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, prompting necrotic exfoliation of the tissue. The degenerated and necrotic endometrium is mixed with blood and discharged to form menstrual blood. The normal thickness of the endometrium can reach 8 to 10 mm. the factors affecting the measurement of the endometrium by ultrasound increase at this stage and the error increases. It is usually considered that an endometrial thickness of 8 to 10 mm is more suitable for conception. The endometrium is thickest during ovulation, so if your menstrual cycle is 28 days, you should have an ultrasound to check the thickness of the endometrium from the 8th, 10th, 12th and 14th days of your period. If the thickness of the endometrium does not reach 7mm by the 14th day of ovulation, it means that your endometrium is thin, and if the thickness of the endometrium is greater than 11mm with enhanced echogenicity, it means that your endometrium is Both are not conducive to conception.