Breast development is a vital part of puberty 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, 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.
Breast 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, a nipple 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 15-24 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 puberty, 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. Generally, there are five stages of breast development in girls.
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.