Recently, several cases of breast cancer during lactation have been found in the clinic, with the development of the two-child policy and the increase of high-collar pregnant women, breast cancer during lactation is becoming more and more negligible and we call for attention to breast cancer during lactation! The incidence of breast cancer is reported abroad to be about 0.2-0.3% of all pregnant women and 2-3% of all breast cancer patients, while domestic reports are higher than foreign levels, accounting for about 7-12% of all breast patients. Generally speaking, primary breast cancer that occurs at the same time of pregnancy or within one year of pregnancy is called gestational breast cancer, while primary breast cancer that occurs during breastfeeding is called lactational breast cancer. This type of breast cancer is less common clinically, but should not be taken lightly. Breast cancer during breastfeeding is mostly seen in women under 35 years old. Due to the change of hormone level in the body, i.e. estrogen and adrenocorticotropic hormone increase significantly, prolactin and growth hormone level increase during lactation, and the decrease of T-cell count in the blood, the immune function of the body decreases, resulting in faster growth and higher malignancy of the tumor; meanwhile, due to the increase of capillaries in the breast and expansion of congestion during pregnancy and lactation, the tumor can spread easily. In addition, due to the increase of breast parenchyma and glandular hyperplasia, the physiological enlargement and hardness of breasts make the tumor not easy to be found at an early stage, and once found, most of them are in the middle and late stage. Therefore, we must be alert to breast cancer during lactation. The symptoms of breast cancer during lactation are similar to those of breast cancer in general, mainly lumps and nipple drainage in the breast, which is usually a single duct in one breast. The lump is often accompanied by enlarged axillary lymph nodes. Due to the enlargement of the breast and the increased density of the breast tissue, X-ray examination often does not easily reveal the tumor, making early diagnosis difficult.