Specifically, the production of two hormones, estrogen and progesterone, signal the development of the glandular breast tissue. This initial growth of the breast may be somewhat painful for some girls. During this time, fat and fibrous breast tissue becomes more elastic. Menstruation prepares the breasts and ovaries for potential pregnancy. The size and shape of women's breasts varies considerably.
Some women have a large amount of breast tissue, and therefore, have large breasts.
Other women have a smaller amount of tissue with little breast fat. Usually, one breast is slightly larger or smaller, higher or lower, or shaped differently than the other. The size and characteristics of the nipple also vary greater from one woman to another. In some women, the nipples are constantly erect. In others, they will only become erect when stimulated by cold or touch.