Endometrial thickness of 10mm should be normal. The thickness of the endometrium is determined by the clinical menstrual cycle. In general, the thickness of the endometrium is determined with ovulation and conception. The normal endometrium is the thinnest when the menstruation is just clean, that is, during the follicular growth phase, at about 4mm. After ovulation, the endometrium continues to thicken under the action of progesterone. If there is no pregnancy, the endometrium starts to peel off before menstruation, and after menstruation, the endometrium returns to about 4mm at the time of clean menstruation.