The breast doesn't have muscles. Breast tissue is situated in addition to the muscles in the upper body wall. Blood vessels and lymphatic vessels (a program of vessels that drains fluid) are located all over the breast. Pregnancy causes elevated amounts of the hormone prolactin, which has a critical role https://grok.com/share/c2hhcmQtMw%3D%3D_21b4ddf4-c274-418a-a078-c9d2af4898ff