Menstruation has not yet arrived, but the breasts are already in pain—this is a common annoyance for many women. Many women experience swelling, hardening, and even pain in their breasts before the onset of menstruation. In severe cases, even slight vibrations or bumps can cause unbearable breast pain. What causes this? It all starts with a mysterious substance—hormones. Each menstrual cycle, levels of estrogen and progesterone rise, causing the mammary ducts and lobular duct endothelial cells to enlarge, and the ductal epithelial cells and stromal cells to become congested and swollen, leading to pain. Thus, different degrees of pain occur before menstruation.