The thickness of the uterine lining on the first day of menstruation is around 0.3-0.6cm, which is normal, and varies due to differences in each person’s physical condition. At the end of menstruation, the estrogen and progesterone levels in a woman’s body are very low, and the lining of the uterus is at its thinnest state, usually between 0.3-0.6cm in thickness. With the growth of estrogen level, the lining will gradually become thicker, and when the endometrium thickens to a certain extent, it will fall off and form menstruation. If the endometrium is thicker, reaching 0.9cm or even 1cm or more on the first day of clean menstruation, it means that the endometrium is likely to be diseased, and diagnostic scraping is needed to find out the condition of the endometrium and to treat it. If the endometrium is thin, it may be related to low estrogen level, and sex hormone six test is needed to clarify the cause and then treat it. Therefore, on the first day of clean menstruation, if the thickness of the endometrium is between 0.3-0.6cm upon examination, it is all normal.