Breast cancer is the most common cancer type during with pregnancy. The incidence of gestational breast cancer is approximately one in 3000 pregnancies, and 0.2 to 3.8 percent of breast cancers occur during pregnancy. Due to physiological changes in the breast (engorgement, hypertrophy), the diagnosis of breast cancer is more difficult in pregnant and postpartum women. Delays in diagnosis are common and adversely impact the outcome. Also the larger tumors at diagnosis in pregnant women are likely to be a consequence of this delay. Breast cancer staging and treatment are possible during pregnancy. Treatment modalities including surgery, chemotherapy and radiotherapy can be executed, dependent on the gestational age. The prognosis is similar to non-pregnant women provided the same treatment is administered. In general, women should be treated according to guidelines for non-pregnant patients, with some modifications to protect the fetus. Trastuzumab is contraindicated during third trimester, radiotherapy during early third trimester must be discussed and fetal dosage must be calculated by phantom models. Informed consent is a critical component of choosing the appropriate therapy. After breast conservative surgery followed by radiation, ductal changes affect the ability to breastfeed. Most women who have undergone irradiation for breast cancer are able to produce milk on the affected side, but the amount of milk produced may be less than that in a non-irradiated breast. Breastfeeding during chemotherapy is contraindicated.