The thickness of the endometrium is usually within the normal range of 4 to 18 mm. The endometrial thickness is 12mm, which is also within the normal range. The thickness of the endometrium is affected by many factors. Generally speaking, as the ovaries mature, the cycle of ovulation, and the menstrual cycle changes, there will be some changes in the endometrium. During menstruation, due to endometrial exfoliation, the endometrium is the thinnest is about 4mm at this time. The thickness of the endometrium depends on the period of menstruation to determine its thickness.