Both the mammary gland and the uterus are effectors of the sex hormone cycle produced by the human hypothalamus-pituitary-ovary endocrine axis, which also appears to change cyclically in synchrony with the uterus. During the follicular phase, the level of estrogen rises, causing the epithelium of the breast ducts to proliferate, and the level of progesterone and lactogen rises during the luteal phase, causing the epithelium in the breast follicles to proliferate and the mammary glands to become engorged with blood until 3-4 days before menstruation, which is more obvious clinically, when the patient feels distension and pain in both breasts and the mammary glands thicken on physical examination. During menstruation, estrogen and progesterone levels decrease rapidly, the breast ducts and glandular epithelium atrophy and partially fall off, and the breast structure returns to the state of ovulation. Clinically, the swelling and pain in the breast can be partially or completely relieved, but this recovery often does not fully return to the original state clinically, thus allowing the breast to accumulate some hyperplastic structures in each cycle of change, so that the breast structure shows the heterogeneity of the hyperplastic state. The clinical manifestation is a limited thickening of some breast tissues in the outer upper quadrant with nodularity and a tough texture, which is evident during menstruation and decreases after menstruation. Therefore, within one week of menstruation is an appropriate time for clinical breast examination. For suspicious thickening of the breast, if there is no obvious evidence of malignancy, follow-up observation can be ordered.