Medical visualization reconstructed from human data of a cross-section of a prepubescent female breast. Breast development begins during fetal development when the beginnings of the milk-duct system are formed and continues as a girl begins puberty when the breasts enlarge and become padded with fat the secretory glands are formed and the duct system matures. Further changes to the breasts occur during pregnancy and menopause. Human females are the only mammals who develop full breasts irregardless of the need for milk production.