Breast development is a vital part of reproduction in the human female. Unlike other mammals, however, human females 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, again at puberty, and during the childbearing years. Changes also occur to the breasts during the menstrual cycle and when a woman reaches menopause.
Breasts begin to form during fetal development with a thickening in the chest area called the mammary ridge or milk line. By the time a female infant is born, nipples and the beginnings of the milk-duct system have formed.
Breast changes continue to occur over the lifespan, with lobes, or small subdivisions of breast tissue, developing first. Mammary glands develop next and consist of 15 to 24 lobes. Mammary glands are influenced by hormones activated in puberty. Involution or shrinkage of the milk ducts is the final major change that occurs within the breast tissue. A gradual shrinking of the mammary glands (involution) typically begins around the age of 35.
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 begins to accumulate causing the breasts to enlarge. The duct system also begins to grow. Usually the onset of these breast changes is also accompanied by the appearance of pubic hair and hair under the arms.
Once ovulation and menstruation begin, the maturing of the breasts begins 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
(Preadolescent) only the tip of the nipple is raised
Buds appear, breast and nipple raised, and the areola (dark area of skin that surrounds the nipple) enlarges
Breasts are slightly larger with glandular breast tissue present
The areola and nipple become raised and form a second mound above the rest of the breast
Mature adult breast; the breast becomes rounded and only the nipple is raised
Each month, women experience fluctuations in hormones that make up 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 formation of the milk glands. These hormones are believed to be responsible for the cyclical changes such as the swelling, pain, and tenderness that many women experience in their breasts just before menstruation.
During menstruation, many women also experience changes in breast texture, with breasts feeling particularly 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.
Many doctors 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 hormone progesterone. In addition, the areolas (the dark areas of skin that surround the nipples of the breasts) begin to swell 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 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.
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. 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 taking hormone therapy may experience some of the premenstrual breast symptoms that they experienced while they were still menstruating, which can include tenderness and swelling. However, if there was sagging of the breasts before menopause, this is not reversed with hormone therapy.