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Development of the breast

The development of the breast occurs in three major phases in our lives: in utero, at puberty, and during pregnancy.

The first stage of breast development starts at about six weeks of fetal development with a thickening called the mammary ridge or the milk line. By six months of fetal development, this milk line extends all the way down to the groin. At this stage, solid columns of cells begin to form each breast bud, with each column developing a separate sweat gland. Each of these columns has its own separate duct leading to the nipple. At the final stage of fetal development, these columns have become hollow and by the time a female baby is born, a nipple and the beginnings of the milk-duct system have formed.

As a girl reaches her adolescence, the first signs of external breast development start to appear. This begins when the ovaries begin to secrete estrogen. Fat in the connective tissue starts to accumulate causing the breast to enlarge. At this stage, the duct system also begins to grow. Usually the onset of breast development coincides with the appearance of pubic hair and hair under the arms. Once ovulation and menstruation begin, the maturing of the breasts starts with the formation of secretory glands at the end of the milk ducts. Then, the breasts and duct system continue to grow and mature, with the development of many glands and lobules. The rate of these growths varies greatly and is different for each individual girl.

Every month, women experience hormone fluctuations from their normal menstrual cycle. Estrogen produced by the ovaries in the first half of the normal menstrual cycle, stimulates the growth of milk ducts in the breasts. The increasing of estrogen level lead to ovulation at the second half of women’s cycle. After ovulation, progesterone increases and takes over the second half of the cycle. The increase in progesterone levels, stimulates the formation of the milk glands. These hormonal changes are believed to be responsible for the recurring changes such as swelling, pain, and tenderness that women experience in their breast just before menstruation.

Many women also experience changes during menstruation in their breast texture (feeling particularly lumpy). The size of the breasts is larger than normal during their period. These are because the glands in the breast growing to prepare for a possible pregnancy. However, if pregnancy does not occur, the breasts return to normal size. Once menstruation begins, the cycle begins again.

Many scientists and doctors believe that the breasts are fully mature when a woman has given birth and produced milk. Breast changes due to pregnancy hormones and progesterone, are one of the earliest signs of pregnancy. Because of the growth of the milk duct system and the formation of the many more lobules, most pregnant women experience tenderness down the sides of the breasts and soreness of the nipples. Normally, the breasts are fully capable of producing milk by the fifth or sixth month of pregnancy. Estrogen controls the growth of the ducts and progesterone controls the growth of the glandular buds. Other pregnancy hormones, such as follicle stimulating hormone (FSH), Luteinizing hormone (LH), prolactin, oxytocin and human placental lactogen (HPL) also play important roles in milk production. Other physical breast changes, such as enlargement and darkening of the areola (black cycle around the breasts) appear. These changes are important in preparation for breastfeeding.