Most of the human female breast is actually adipose tissue (fat) and connective tissue, rather than the mammary glands. There is naturally a great variety in the size and shape of breasts in women (and men), with size being affected by various factors including genetics.
The primary anatomical support for the breasts is thought to be provided by the Cooper's ligaments, with additional support from the skin covering the breasts themselves, and it is this support which determines the shape of the breasts. The breasts naturally sag through ageing, as the ligaments become elongated. This process may be accelerated by high impact exercises, and a brassiere may reduce this effect by providing external support, although the health benefits of wearing of a brassiere are not universally accepted. Sagging breasts (ptosis) are considered undesirable by some, and some older women seek cosmetic surgery to raise their busts.
As breasts are mostly composed of adipose tissue, their size can change over time if the woman gains or loses weight. It is also typical for them to grow in size during pregnancy and whilst breastfeeding, mainly due to hypertrophy of the mammary gland in response to the hormone prolactin. The size of a woman's breasts usually fluctuates during the menstrual cycle, particularly with premenstrual water retention. An increase in breast size is also a common side effect of use of the contraceptive pill.
There is no relationship between breast size and ability to breastfeed, and it is a common misconception that human female breasts are shaped the way they are so that they can feed babies by producing milk - their shape is thought to have evolved due to sexual attraction.
The size of a woman's breasts is typically expressed as a "bra size". According to the results of the "Size UK" survey, the average bra size in the UK has increased from a 34B in the 1950s to a 36C today, and the average size for U.S. women is a 34B as of 2005 by the CDC. Women with exceptionally large breasts may experience back pain, whilst in some western societies there is a belief amongst some that small breasts make a woman less sexually attractive.
Some women suffer from insecurity about their breasts, and in some cultures a number of women who are unhappy with their size seek surgery either to artificially reduce or enlarge their breasts. In the United States of America, 427,574 such surgeries were performed in 1973. Some women in developed countries undergo breast reconstruction after mastectomy for breast cancer, a result of the high value placed on symmetry of the female human form in those cultures, and because women often identify their femininity and sense of self with their breasts.
It is typical for a woman's breasts to be unequal in size (statistically it is slightly more common for the left breast to be the larger), particularly whilst the breasts are developing during puberty. In some rare cases, one breast may be greatly larger or smaller than the other, or fail to develop entirely.