Breast tissue is affected by nearly every hormone produced by the body. Hormones from the ovaries, adrenal glands, pituitary gland, and brain (hypothalamus) work together to influence breast growth and function beginning before birth and continuing throughout a woman's life. The most important hormone causing breast enlargement is estrogen, which is released by the ovaries. Estrogen triggers growth in breast tissue by attaching to cells at points called estrogen receptors.

Women with overly large breasts usually have normal blood levels of estrogen and the normal number of estrogen receptors. Therefore, the main scientific theory explaining excessive breast enlargement is that breast tissue in some women is hypersensitive to estrogen. You might guess that it is the gland tissue in the breast that responds the most to estrogen, but in fact it is the fibrous tissue and fat that are most sensitive to the hormone.

The important point here is that breast enlargement for the majority of potential breast reduction patients is due to increases in hormone-sensitive breast fat and fibrous tissue and is not due to increased body fat that happens to be in the breast and that can be expected to shrink with weight loss.

The first big change in breast size occurs during puberty. Girls destined to become large-breasted are usually the first in their class to start developing breasts. These girls may have breast buds as early as age eight, and breast development can precede other obvious signs of puberty by as much as two years. Breast enlargement may continue even after a teenage girl reaches her adult height. Dramatic and often painful massive breast enlargement during the hormone stimulation of adolescence is called gigantomastia.

Pregnancy causes a woman's breasts to enlarge, and they remain enlarged until lactation ceases. Once a woman stops nursing (or if she doesn't nurse at all), the milk glands and the increased blood flow in her breasts start to shrink. These effects are the result of changes in hormone levels and take six months to a year to be complete. There is tremendous variation among women in the permanent effects of pregnancy and nursing on breast size.

A particular woman's final breast size and shape after pregnancy are influenced both by how much her breasts responded to her temporarily elevated hormone levels and by her skin tone. Some women experience permanent breast enlargement that does not respond to weight loss. These women often find that their breast size progressively increases with each pregnancy

 Other women's breasts return to approximately their prepregnancy size and shape. Still other women lose much of their original breast fullness, due to shrinkage of the glandular breast tissue. Some of these women have poor skin elasticity and are left with elongated, flat, drooping breasts. Even these women can have significant symptoms related to their breast size, since the excess skin is heavy and does not lessen with weight loss.

By the time a woman reaches the age of thirty-five, her ovarian hormone production starts to drop, and the glandular portions of her breasts start to shrink further. After menopause, when her ovarian hormone production stops completely, her breast ducts and glands degenerate until her breasts are made up almost entirely of skin, fat, and connective tissue.

Hormones taken as medication can cause breast enlargement or can reduce the degree of breast tissue shrinkage that normally occurs during menopause. Both birth control pills (taken for contraception or to regulate the menstrual cycle) and hormone replacement therapy (HRT) for menopausal symptoms can cause breast enlargement.