Breast cancer is generally believed to carry a worse prognosis during pregnancy because of the potential adverse effects of anticancer treatments on the fetus and of pregnancy related hormonal and immunological modifications on the disease. Problems arise both with breast cancers diagnosed during or immediately after pregnancy and with pregnancies occurring in patients previously treated for breast cancer. The incidence of these problems is difficult to estimate, in part because pregnancy-related changes in the breasts obscure clinical and radiological manifestations. Prognosis depends more on patient age than on gestational age at diagnosis. Other prognostic factors include size of the tumor and node involvement. Although the outcome of N+ tumors is more severe during pregnancy, overall the poor prognosis of breast cancer during pregnancy is due to the young age of patients and pregnancy has no bearing on prognosis after adjustment for stage. Similarly, women who become pregnant after being treated for breast cancer do not have worse outcomes. Management is difficult because some treatments cannot be used during pregnancy; the approach is similar to that used in non-pregnant women, although chemotherapy should be given only after 14 or 15 weeks gestational age and irradiation therapy only after delivery.