We often encounter women who come for follow-up because of a physical examination or other reasons for finding a thick endometrium. In fact, in women of normal reproductive age, due to the influence of estrogen and progesterone in the blood, the thickness and structure of the endometrium will change periodically with the menstrual cycle, and its ultrasound sonogram will also change accordingly. The thickness of the endometrium in the hyperplastic phase is generally considered to be 4-8 mm, with isoechoic or slightly enhanced echogenicity compared to the middle and outer layers of the uterus. In late hyperplasia or early ovulation, multiple layers of endometrial structures can be seen in the form of a “trilinear sign”. The middle line is the cavity line and the innermost hypoechoic layer is the endometrial dense layer with edema. After ovulation, the endometrium shows a secretory response, with a medium to high echogenicity, and the endometrium may be thicker at this time, up to 14 mm or more. After menopause, due to the atrophy of the endometrium, the high echogenicity of the endometrium in connection with the endometrial muscle layer is not clearly visible; this layer is often destroyed in the presence of adenomyoma or endometrial cancer. Therefore, it is recommended that women who do not have any menstrual abnormalities do not need to dwell on their slightly thickened endometrium. As an ultrasonographer we evaluate a patient’s endometrium not only by its thickness but also by whether the internal structure is consistent with the medical history.