Whether or not endometrial thickening of 18mm is serious depends on the specific cause of the endometrial thickening. If it is simply due to high estrogen level, it is usually not serious; if it is due to malignant lesions, it needs to be taken seriously. 1. Simply high estrogen level: If the endometrial thickening reaches 18mm due to simply high estrogen level, the pathology suggests simple hyperplasia or complex hyperplasia, which is not too serious in most cases. In most cases, it is not too serious. Progesterone-related treatment is sufficient in such cases. 2. Endometrial malignant lesions: if the pathology result suggests atypical hyperplasia of the endometrium, this is the pre-malignant endometrial lesions, and this situation needs to be taken seriously, because the atypical hyperplasia has a high possibility of developing into endometrial cancer. Therefore, if the endometrial thickening reaches 18mm, it is necessary to find out the cause of the disease under the guidance of the doctor in time before judging its seriousness.