Breast cancer during pregnancy and lactation. Breast cancer is the most frequently seen cancer in pregnancy and lactation, but the incidence is low, the disease being seen in approximately 0.03% of pregnancies. Only 1% to 2% of breast cancer overall is diagnosed during pregnancy or lactation. There is no evidence to implicate pregnancy or lactation in either the etiology or the progression of breast cancer. Careful breast examination early in the pregnancy is very important to find solid masses that require biopsy before breast engorgement hides them. Therapeutic options vary, depending on the stage of disease and the stage of the pregnancy. Operable disease in the first 6 to 7 months of the pregnancy should be treated by mastectomy, as irradiation is contraindicated. Late in the pregnancy, a lumpectomy and axillary dissection can be done, with irradiation being delayed until after delivery. General anesthesia is safe if the usual precautions are taken to compensate for the physiologic changes induced by pregnancy. Unfortunately, delay in diagnosis is common, and 70% to 89% of patients with operable primary lesions have positive axillary lymph nodes. Late stage appears to be the only reason for the generally worse prognosis in these patients, as stage for stage, they have a course similar to that of nonpregnant patients. Adjuvant chemotherapy can be considered late in the pregnancy but should usually be delayed until after delivery. In patients with locally advanced or metastatic cancer diagnosed early in the pregnancy, for whom both chemotherapy and radiation therapy would normally be recommended, consideration must be given to termination of the pregnancy. There is no evidence that termination of pregnancy improves the outlook for the patients, but it does permit standard aggressive therapy in advanced disease. 