The endometrial thickness of 21mm is clinically determined to be severe or not based on the menstrual cycle. The thickness of the endometrium 21mm is usually measured by imaging, which indicates the thickness of the endometrium by ultrasonography or MRI. The thickness of the endometrium 21mm depends on what period of menstruation it is in and whether there is vaginal bleeding. If it is after menstrual ovulation, 21mm of endometrial lining before menstruation is within the normal range. As the menstrual cycle approaches, the lining peels off and the endometrial lining may become thinner. If the endometrium is 21mm just after menstruation, i.e. during the follicular phase, it is an abnormality, so the thickness of the endometrium is determined by the menstrual cycle. The normal endometrium varies with the menstrual cycle. After menstruation, the endometrium is at its thinnest state, usually about 4mm. Before menstruation, i.e. after ovulation, the endometrium gradually thickens and may reach about 18-20mm before menstruation. Therefore, the thickness of the endometrium is determined according to the menstrual cycle, and if the thick endometrium is combined with vaginal bleeding, then the thick endometrium is an abnormal condition. Clinically, it is not possible to decide whether the endometrium is normal or abnormal simply based on the thickness of the endometrium, but based on the performance during the menstrual cycle and the menstrual bleeding.