Ultrasound can check the endometrial thickness, and the thickness and echogenicity of the endometrium will change with the menstrual cycle, which is divided into the following three phases: 1. Menstrual phase: the endometrium is exfoliated and a liquid dark area can be seen in the uterine cavity; 2. Proliferative phase: the endometrium is linear and hyperechoic; in the early and mid-proliferative phase, the endometrial thickness is usually 1-3 mm, and in the late proliferative phase the endometrium can reach 3-5 mm; 3. Secretory phase: the endometrium thickens and begins to show trilinear-like changes, i.e., the hypoechoic functional layer and the hyperechoic basal layer constitute the trilinear sign; in the early and middle stages of secretion, the thickness of the endometrium can reach 5-7mm, and in the late stage of secretion, the endometrium thickens and the echogenicity is enhanced, and the thickness of the endometrium can reach 10mm.