Breast cancer in pregnancy is a neoplasm diagnosed during the first year postpartum or during lactation. Effect of pregnancy on the neoplastic process is not fully understood. The diagnosis is based on the tests performed as in case of non-pregnant women with the exception of the use of ionizing radiation imaging techniques and use of contrast agent. The course of cancer treatment is the same as for other patients, but mastectomy is recommended after the first trimester of pregnancy. Conservative treatment is not recommend due to contraindications for radiotherapy during pregnancy. The studies have demonstrated that chemotherapy regimens based on anthracycline and cyclophosphamide are safe. Other cytotoxic drugs and hormonal therapy are contraindicated because of severe damage to the fetus. Adjunctive therapy in the form of antiemetics, analgesics is permitted, but under certain conditions. Breast-feeding after labour is not recommended during chemotherapy, hormonal therapy and biological therapy, because many drugs are excreted into breast milk. Definite effect of pregnancy on breast cancer prognosis has not been established. Since changes in the mammary gland during pregnancy and lactation are considered as physiological, it happens that diagnosis is delayed even up to 7 months. It is believed that abortion does not improve treatment results. Another pregnancy can be considered only after 3 years when anticancer treatment is finished, cancer relapse and distant metastases are ruled out