The breast serves a vital purpose in the reproductive role of human female, and secondary role in gender identification. Unlike other mammals, however, human females develop full breasts long before they are needed to nurse their offspring. This is due in part to the breast’s role in sexual identification.

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, pregnancy, and menopause.

Breast changes are closely related to age. Approaching and during the age of maturity, these developments fit into three stages:

• lobule development, which takes place between the ages of 10 and 25;
• glandular development, which is under the influence of menstrual hormones and occurs between the ages of about 13 and 45;
• and involution, or shrinkage of the milk ducts, which begins from about age 35.

Beginning breast development
This first stage of development takes place at about six weeks of fetal gestation and accelerates rapidly over the months thereafter. By the third trimester, cells in the unborn female have aligned into the basic structures of the breast. By the time an infant girl is born, a nipple and the beginning of the milk-duct system have formed.

Changes occur in at puberty
As a girl approaches adolescence, the first outward signs of breast development begin to appear. When the ovaries start to secrete estrogen, fat in the connective tissue responds to this hormone stimulation and begins to accumulate, causing the breasts to enlarge. The duct system also begins to grow, and progesterone stimulates the growth of the glandular buds. Usually the onset of these breast changes is also accompanied by the appearance of pubic hair, hair under the arms, overall physical growth, and other aspects of physical and sexual maturation.

Once ovulation and menstruation begin, the maturing of the breasts is underway with the formation of secretory glands at the end of the milk ducts. The breasts and duct system continue to grow and mature, with the development of many glands and lobules. The rate at which breasts grow varies greatly and is different for each young woman.

Female Breast Developmental Stages
Stage 1	(Preadolescent) only the tip of the nipple is raised
Stage 2	buds appear, breast and nipple raised, and the areola (dark area of skin that surrounds the nipple) enlarges
Stage 3	breasts are slightly larger with glandular breast tissue present
Stage 4	the areola and nipple become raised and form a second mound above the rest of the breast
Stage 5	mature adult breast; the breast becomes rounded and only the nipple is raised

Cyclical changes during menstruation
Each month, women experience fluctuations in hormones during the normal menstrual cycle. Estrogen, which is produced by the ovaries in the first half of the menstrual cycle, stimulates the growth of milk ducts in the breasts. The increasing level of estrogen leads to ovulation halfway through the cycle, and then the hormone progesterone takes over in the second half of the cycle, stimulating the milk glands. These hormones are believed to be responsible for the cyclical changes such as the swelling, pain, and tenderness that most women experience in their breasts just before menstruation. Breast pain or mastalgia is a common, benign condition.

During menstruation, many women also experience changes in breast texture, with breasts feeling lumpy. These are the glands in the breast enlarging to prepare for a possible pregnancy. If pregnancy does not occur, the breasts return to normal size. Once menstruation begins, the cycle begins again.

Included among common benign breasts conditions that younger women may experience are lumps called fibroadenomas.

Pregnancy and lactation
Many physicians believe the breasts are not fully mature until a woman has given birth and produced milk. Breast changes are one of the earliest signs of pregnancy – a result of the pregnancy hormone, progesterone. In addition, the areolas (the dark areas of skin that surround the nipples of the breasts) begin to expand, followed by the rapid swelling of the breasts themselves. Most pregnant women experience tenderness down the sides of the breasts and tingling or soreness of the nipples because of the growth of the milk duct system and the formation of the many more lobules.

By the fifth or sixth month of pregnancy, the breasts are fully capable of producing milk. As in puberty, estrogen controls the growth of the ducts and progesterone controls the growth of the glandular buds. Many other hormones, such as follicle stimulating hormone (FSH), luteinizing hormone (LH), prolactin, oxytocin, and human placental lactogen (HPL) also play vital roles in milk production.

Other physical changes, such as the prominence of the blood vessels in the breast and the enlargement and darkening of the areola occur. All of these changes are in preparation for breastfeeding the baby after birth.

Lactation can also sometimes result in abnormal nipple discharge, and it increases the risk of breast pain or infection or inflammation, although nonlactating women can also experience these benign conditions of the breast.

Menopause
By the time a woman reaches her late 40s and early 50s, menopause is beginning or is well underway. At this time, the levels of estrogen and progesterone begin to fluctuate, with levels of estrogen dramatically decreasing over time. This leads to many of the symptoms commonly associated with menopause.

With this reduction in the stimulation by estrogen to all tissues of the body, including the breast tissue, there is a reduction in the glandular tissue of the breasts. Without estrogen, the connective tissue of the breast becomes dehydrated and inelastic, and the breast tissue, which was prepared to make milk, shrinks and loses shape. This leads to the "sagging" of the breasts often associated with women of this age.

Women who are on hormone replacement therapy may experience some of the premenstrual breast symptoms that they experienced while they were still menstruating, which can include tenderness and swelling in the breasts. However, sagging of the breasts before menopause is not reversed with hormone replacement therapy.

Women in or approaching this age group are also more likely to develop cysts in their breasts or abnormal nipple discharge.