An endometrial thickness of 12mm is the thickness of the lining of the uterus measured by ultrasound, which is able to infer the approximate menstrual cycle and whether there are any lesions. The endometrium is the pink mucous membrane tissue that covers the inside of the myometrium. Starting from puberty, the thickness of the endometrium changes with the hormonal cycle, i.e., the onset of menstruation, thickening of the endometrial secretion before menstruation, and shedding of the endometrium after menstruation to become thinner, and the endometrium of a normal human being is around 5~10mm. Before menstruation, due to the thickening of endometrial secretion, 12mm is normal, but after menstruation, 12mm of endometrial is abnormal, and you need to go to gynecology department in time to receive checkups and relevant treatments.