Sudden enlargement and swelling of breasts may be related to menstruation, pregnancy, breast enlargement, breast cancer and other diseases. 1. Menstruation: The estrogen level in a woman’s body increases before the onset of menstruation, which will cause some stimulation to the breast tissue and lead to breast enlargement and swelling. 2. Pregnancy: When a woman is pregnant, the level of estrogen and progesterone in her body will also change, which will stimulate the breast tissue and cause sudden enlargement and swelling of the breasts. 3. Breast hyperplasia: The imbalance of estrogen-progestin ratio in the body, the abnormal quality and quantity of breast sex hormone receptors, and the elevation of prolactin will affect the normal growth of mammary glands, resulting in sudden enlargement and swelling of the breasts. 4. Breast Cancer: Women aged 30 to 50 who have sudden breast enlargement and swelling pain must be alert to breast cancer. The incidence of breast cancer is mostly related to endocrine dysfunction. During the period of illness, the malignant tissues continue to proliferate and expand, which can lead to the enlargement and state of the breast. There may be other reasons such as mastitis, breast trauma, etc., which may cause sudden enlargement and swelling of breasts. It is recommended to go to the breast surgery department of the hospital as soon as possible and cooperate with the doctor to receive treatment.