My sister's newborn daughter has enlarged breasts.
Breast development is influenced by the sex hormone estrogen.
The levels of estrogen vary significantly at different times during life. And its effects cause changes throughout the body. These changes include maturation of mucous membranes of the vagina, breast enlargement, and bone maturation. The fetus grows in an environment rich with hormones. This is due to the levels of estrogen being about as high as a normal girl going through puberty.