12 weeks pregnant left breast swelling and pain, may be physiological reasons, may also be breast disease, early pregnancy breast swelling and pain, enlargement belongs to the physiological reasons, you can choose comfortable underwear, massage breasts to help relieve; if the breast swelling and pain accompanied by mammary gland overflow, lumps and so on, may suggest mastitis, breast cancer and other diseases, need to consult a doctor in time. 1. Physiological reasons: 12 weeks of pregnancy belongs to the early stages of pregnancy, due to the placenta, chorionic secretion of a large number of estrogen and progesterone, these hormones will lead to mammary gland hyperplasia, breast enlargement, breast swelling and pain, which is a normal phenomenon, you can choose the appropriate and comfortable underwear, gentle massage of the breasts, and appropriate rest to relieve discomfort. 2. Breast disease: In addition to breast swelling and pain, it may also be manifested as mammary gland overflow, bloody secretion, local redness, swelling, fever, lymph node enlargement, lumps and other manifestations, such as mastitis need to comply with the doctor’s instructions to use antibiotics such as cefixime treatment, if the breast cancer needs to be operated combined with radiotherapy treatment. Therefore, the left breast swelling and pain in the 12th week of pregnancy should first be considered physiological causes, but if abnormal breast overflow, lymph node enlargement and other abnormalities are observed, it is necessary to consult a doctor in time for evaluation.