The endometrium undergoes 3 phases of cyclic changes during the menstrual cycle The endometrium is divided into a basal layer and a functional layer. The functional layer of the endometrium is regulated by ovarian hormonal changes and has cyclic proliferative, secretory and exfoliative changes; the basal layer regenerates and repairs endometrial trauma after menstruation. According to the histological changes of endometrium, the menstrual cycle is divided into 3 phases: proliferative phase, secretory phase and menstrual phase. 1. The proliferative phase The 5th to 14th day of the menstrual cycle corresponds to the follicular phase of the ovarian cycle. Under the action of estrogen, the epithelium, glands, interstitium and blood vessels on the surface of the endometrium show proliferative changes, which is called the proliferative phase. During this period, the endometrium is gradually thickened. In the early proliferative phase, the endometrium is 3-6 mm thick, and when the follicles reach maturity, the endometrium is generally 10-14 mm thick. 2.Secretory period The 15th~28th day of menstrual cycle corresponds to the luteal phase in the ovarian cycle. The luteal secretion of estrogen and progesterone causes the endometrium to continue to thicken during the proliferative phase, which is rich in nutrients and is conducive to the fertilization of eggs, called the secretory phase. In the late secretory phase, the thickness of endometrium can reach 10 mm. 3.Menstrual period The 1st~4th day of menstrual cycle is the period when the spongy functional layer of endometrium disintegrates and sheds from the basal layer. In the 24 hours before menstruation, the endometrial spiral arteries contract and diastole rhythmically, resulting in ischemic necrosis and exfoliation of the distal vessel walls and tissues, and the shed endometrial fragments and blood flow out of the vagina together, i.e., menstruation. Two to three days after menstruation, the endometrial thickness is usually 5 to 6 mm. There are 3 possibilities for endometrial thickening and uneven echogenicity 1. Endometrial hyperplasia including simple hyperplasia, complex hyperplasia and atypical hyperplasia, which can be caused by incomplete shedding of endometrium during menstruation. The shedding of endometrial parts before normal menstruation is synchronized, complete and rapid, while in the absence of ovulation the endometrium is irregularly and incompletely shed due to fluctuations in estrogen, lacking sufficient functional layers of tissue lost and difficult to effectively stimulate the regeneration and repair of the endometrium, which can be manifested on ultrasound images as endometrial echogenic inhomogeneity. 2, very small endometrial polyps Endometrial polyps are formed by inflammation and other factors, and consist of endometrial glands and interstitial masses, often with tips protruding into the uterine cavity. Patients with endometrial polyps may experience menstrual changes, including increased menstrual flow, prolonged periods, irregular vaginal bleeding, etc. Ultrasound sonograms show enhanced echogenicity and clear demarcation with the endometrium. However, if the polyp is small, the patient may not have any clinical symptoms, and only the presence of endometrial polyp is found during pathological examination. 3.Endometrial cancer is a group of epithelial malignant tumors occurring in the endometrium, and endometrial adenocarcinoma is the most common. It is most common in endometrial adenocarcinoma. The ultrasound sonogram shows parenchymal inhomogeneous echogenicity in the uterine cavity and uneven endometrium. Patients with endometrial cancer have no obvious symptoms in the very early stage, and as the disease progresses, symptoms such as vaginal bleeding, vaginal discharge and pain will appear. Ultrasound is the main auxiliary examination for gynecological diseases, but ultrasound examination is only an imaging examination and cannot replace pathological examination. In the early stage of certain diseases when ultrasound presentation is atypical, ultrasound diagnosis is somewhat subjective and different doctors may have different diagnoses. Diagnostic curettage or hysteroscopy can be performed to further clarify the diagnosis.