In general, the endometrium in the follicular and secretory phases is considered thin if its thickness is less than 8mm. Its thickness changes with the menstrual cycle, therefore, the menstrual cycle should also be taken into account in determining whether the endometrium is thin or not. Overall, the thickness of normal endometrium is between 8 and 10mm. Due to the effect of estrogen and progesterone, the thickness of the endometrium changes throughout the menstrual period, and becomes thinnest at 3-6mm during menstruation due to the stripping of the endometrium, and then reaches 8-10mm during the follicular phase, and thickest at 10-13mm during the secretory phase. Normally, uterine ultrasonography is not performed during the menstrual period, and the endometrial thickness of the follicular phase and secretory phase is less than 8mm, which is considered as thinning of the endometrial lining. Patients with thin endometrium need to go to the gynecology department of regular hospitals for gynecological consultation and standardized treatment after relevant examinations.