In general, the thickness of the endometrium before menstruation is 8-13 mm, which is a normal physiological state. The endometrium is divided into three parts: the dense layer, the spongy layer and the basal layer. The functional layer is composed of the dense layer and the spongy layer. Once the functional layer is continuously proliferating under the level of constant hormonal stimulation, it may cause the phenomenon of endometrial thickening. Once the endometrium is thicker than 13mm, it is possible that the hormone level is particularly high. It is recommended to check the hormone level and also to do ultrasound of the uterine adnexa to see if there are any structural lesions or tumors. If it is thin, you should also check your uterine adnexal ultrasound to see what kind of lesions are there and then carry out specific treatment.