The possible reasons for the appearance of veins under the breast include the normal performance of the thin dermis layer of the skin, mastitis, breast hyperplasia, breast tumors and other factors. 1. Normal performance: women with fair skin, the veins under the skin are more obvious, you can see the performance of blue veins, this situation can also be seen in normal women 2. Mastitis: when the patient has mastitis, the internal mammary gland will be congested and edematous, which will cause the subcutaneous venous blood vessels to return poorly, congestion, tortuous, and then appear subcutaneous vein-like manifestations. 3. Mammary gland hyperplasia: especially with patients before and after menstruation, the situation of mammary gland hyperplasia will be aggravated, and at the same time, it will also show the subcutaneous vascular reflux is not smooth, which is manifested as the expansion of the blood vessels in the subcutaneous coarseness. 4. Breast tumor: in early stage, it can be manifested as local vascular expansion, tortuous, ultrasonic examination can also find irregular occupying lesions in the breast. Some patients may have nipple overflow, nipple inversion, orange peel-like appearance and so on. There are many other causes for the appearance of bruises under the breast, such as vascular lesions, and it is recommended to go to the hospital in time to clarify the condition, in order to avoid delaying the best time for diagnosis and treatment.