A centered uterine line is a normal phenomenon and is a common descriptive term used in gynecologic ultrasound. An endometrial thickness of about 15 millimeters is not necessarily a problem, depending on the time of the examination. The menstrual cycle can be divided into proliferative, secretory and menstrual phases. 1. Proliferative phase: refers to the period after the end of menstruation and before ovulation, this period of time the endometrium will gradually thicken under the action of hormones, under normal circumstances, the thickness of the endometrium should be 8~10 millimeters. 2. Secretion period: it refers to the period after ovulation and before the onset of menstruation, the normal thickness of the endometrium should be 10~15 millimeters. 3. Menstrual period: it refers to the period of time when menstruation occurs, due to the shedding of the uterine lining, so the normal thickness of the uterine lining should be within 5 millimeters. So there is no problem with the uterine line being centered, and the endometrial thickness of 15 millimeters depends on the specific time of the examination. It is normal to be in the premenstrual period, but if it is in the proliferative period then it is endometrial thickening, which requires prompt medical attention and a clear diagnosis under the guidance of a specialized doctor.