The breast is a target organ for estrogen. During the menstrual cycle, the glandular tissue of the breast changes accordingly with different hormonal changes at different stages of the menstrual cycle, therefore, the results of breast ultrasound and mammography are also affected by the menstrual cycle. So when is the most appropriate time to have a mammogram? During the first half of the menstrual cycle, the follicles mature and the level of estrogen rises, resulting in proliferative changes in the breast, including stretching of the ducts, epithelial hyperplasia, enlargement of the follicles, enlargement of the duct lumen, periductal edema, increased vascularity and tissue congestion. After ovulation, progesterone levels increase and prolactin also increases, resulting in hypertrophy of ductal epithelial cells in the lobules and increased secretions in the interlobular and terminal ducts. Breast examinations, ultrasound and mammograms done at this time may affect the results and the doctor’s judgment due to physiological changes in the breast. After menstruation, the levels of estrogen and progesterone decrease rapidly, the epithelial cells of the breast ducts secrete less, the cells atrophy and fall off, the edema subsides, and the volume of the lobules and alveoli of the breast shrinks. If there is a tumor on the breast, the results of breast examination, breast ultrasound or mammogram will be more clear. In conclusion, it is better to perform breast examinations after menstruation (usually about 10 days after menstruation). For patients who need regular examinations, it is also recommended that they be performed at the same time after menstruation, so that the results are more comparable.