The endometrium will thicken and shed with the changes of estrogen and progesterone secreted by the ovaries in the body during each physiological cycle to form menstruation. Therefore, the thickness of the endometrium changes during different periods of menstruation, and after menopause, without the effect of estrogen and progesterone, the endometrium undergoes atrophic changes. Taking the normal menstrual cycle of 28 days as an example, the thickness of endometrium in the proliferative phase is growing from 0.5mm to 5mm, the thickness of endometrium in the secretory phase should reach 10mm, the endometrium is shed in the menstrual phase, and then a new menstrual cycle starts again. Thick endometrium in reproductive age is mostly caused by endocrine disorders resulting in endometrial hyperplasia, endometrial polyps, and submucosal fibroids, but it is also necessary to be alert to the possibility of atypical endometrial hyperplasia or even endometrial cancer. The endometrial thickness in menopausal women should be less than 4-5 mm, and if it exceeds this value, diagnostic curettage is required, as it is more important to be alert to the possibility of malignant disease. Therefore, for endometrial 12mm, it depends firstly on the age of the patient, then on which stage of menstruation she is in, and finally on whether it has been so for a long time or if it is the first time it is found. If it is found for the first time, it can be reviewed again after the menstruation is clean, and the persistent thickening will be checked again.