Breast development is a vital part of reproduction in the human female.
Unlike other mammals, however, human females are the only ones who develop full breasts long before they are needed to nurse their offspring. Breast development occurs in distinct stages throughout a woman's life, first before birth, and again at puberty and during the childbearing years. Changes also occur to the breasts during menstruation and when a woman reaches menopause. The development and kinds of breast changes that take place are directly related to age. This first stage of development begins at about six weeks of fetal development with a thickening called the mammary ridge or the milk line.