There is also appreciable variation in size amongst persons. Permanent breast development through puberty is brought on by estrogens along with the growth hormone. Woman human beings are the sole mammals that permanently acquire breasts at puberty; all other mammals acquire their mammary tissue throughout the latter period of pregnancy. https://grok.com/share/c2hhcmQtMw%3D%3D_5ad6450c-533d-442c-a09c-9f7bce1babfb