The endometrium is 3mm thick on the 8th day of menstruation. On the 8th day of menstruation, a woman is in the proliferative phase of the endometrium, and the thickness of the endometrium will start to increase from about 0.5mm during menstruation to a thickness ranging from 3-5mm, which is also known as the follicular phase. Therefore, it is normal for a woman’s endometrial lining to be 3mm thick on the 8th day of menstruation. The thickness of the lining of the uterus is affected by the hormone levels in the body. The thickness of the lining of the uterus is thinnest during menstruation, and then grows gradually after menstruation, and then falls off after a certain level of growth, and so on and so forth. Some women may have abnormal endometrial thickness, which requires ultrasound examination and prompt treatment.