Breast cancer staging and treatment are possible during pregnancy, and should be defined in a multidisciplinary setting. Tumour biology, tumour stage, and gestational stage at diagnosis determine the appropriate approach. Surgery for breast cancer is possible during all trimesters of pregnancy. Radiotherapy is possible during pregnancy but, dependent on the fetal dose received, can result in poor fetal outcomes. The decision to give radiotherapy should be made on an individual basis. Evidence increasingly supports administration of chemotherapy from 14 weeks' gestation onwards. New breast cancer treatments might be applicable to pregnant patients, but tamoxifen and trastuzumab are contraindicated during pregnancy. Cancer treatment during pregnancy will decrease the need for early delivery and thus prematurity, which is a major concern in management of breast cancer in pregnancy.