To check the endometrial thickness should be in 3-7 days after menstruation. If the menstruation is just clean and the endometrium is thicker than 1.0cm or more, it belongs to endometrial thickening, which can be seen as abnormal uterine bleeding, endometrial hyperplasia or endometrial polyp and other diseases, it is recommended to perform diagnostic scraping or hysteroscopy. To check whether the endometrium is thinner, it is important to do so at the time of the upcoming menstruation, because during this period the endometrium is already in a thicker state relative to the period of the upcoming menstruation after the proliferative and secretory phases. If the endometrium is thinner and lower than 0.5cm during this period, it is generally considered as thin endometrium, which can be seen as a result of thin endometrium caused by low hormone level, or endometrial damage caused by excessive scraping or abortion. Therefore, it is better to check the endometrial thickness at different times.