The thickness of the endometrium varies throughout the menstrual cycle. The best time to check the endometrial thickness depends on what the purpose of the examination is: 1. The best time to check the endometrial thinning disease is within 3 days before the menstrual flow, which requires vaginal ultrasound detection. The endometrial thickness is the thickest period in the whole menstrual cycle, because the estrogen level basically reaches its peak, stimulating the endometrial proliferation and secretion, and the impending hormonal withdrawal and triggering menstruation, when The thickness of the endometrium should be 10-12 mm, if it is less than 8 mm it can be considered as too thin endometrium. 2. If the disease of too thick endometrium is checked, vaginal ultrasound can be done within 3 days after menstrual cleansing, if it is more than 8 cm at this time it is considered too thick endometrium. Both thin and thick endometrium are causes of infertility, and targeted treatment needs to be done according to the test results.