Breast development is a vital part of puberty in females. It occurs in stages, first before birth, during puberty, and then again during the childbearing years. Changes also occur to the breasts during menstruation and when a woman reaches menopause.
Breasts start to form during fetal development. This is when a thickening in the chest area called the mammary ridge or milk line develops. By the time a female infant is born, a nipple and the start of the milk-duct system have formed.
Breast changes continue 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 starts around the age of 35.
As a girl reaches puberty, the first outward signs of breast development start to show. When the ovaries start to secrete estrogen, fat in the connective tissue starts to buildup causing the breasts to enlarge. The duct system also starts to grow. Usually, pubic and underarm hair also appear at this time.
Once ovulation and menstruation start, the breasts begin to mature and glands form 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 of breast growth varies and is different for each young woman. Generally, there are 5 stages of breast development in girls.
Female Breast Developmental Stages
Preteen, 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 have hormone changes that make up the normal menstrual cycle. Estrogen is made by the ovaries in the first half of the menstrual cycle. It stimulates the growth of milk ducts in the breasts. The increasing level of estrogen leads to ovulation halfway through the cycle. In the second half of the cycle, the hormone progesterone takes over. It stimulates 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 have in their breasts just before menstruation.
During menstruation, many women also have 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 starts again.
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