The causes of breast swelling and pain when menstruation occurs are related to breast enlargement and changes in estrogen and progesterone. Estrogen and progesterone reach their peak when menstruation occurs. Estrogen promotes the proliferation of breast ducts, while progesterone promotes the development of breast follicles. Therefore, stimulated by a brief rise in hormones, women may experience breast swelling and distension. Some patients who have breast enlargement themselves will experience more pronounced symptoms of breast swelling and pain. When the level of estrogen and progesterone drops significantly after menstruation, this swelling and pain will disappear naturally. Most of these periodic, regular breast pains do not require special treatment, but an annual routine breast ultrasound is very necessary. Because of the high probability of breast disease, regular follow-up is necessary.