Breasts consist of glands, fat and fibrous tissue. The glandular component enlarges during times of hormonal change, such as during puberty, during pregnancy and even premenstrually, when the secretion of estrogen is increased. Fatty tissue will increase and decrease with weight changes. The breast tissue found just underneath the surface of the skin is very elastic and can change in size as the glands expand. However, Cooper's ligaments, which are the fibrous tissues that serve as an anchor and provide support, do not share this quality of elasticity. The elastic tissues can expand and contract, whereas Cooper's ligaments will stretch out but not get back to their original length.

With time, all of this wear and tear on breast tissue can lead to a flattening of the breasts. Use of a bra can provide external support, taking pressure off of the Cooper's ligaments, which helps keep breasts from flattening.

Women with larger breasts will benefit the most from wearing a properly fitted bra. Sometimes these bras have to be specially ordered to fit well. For these women, a bra can provide the extra support that prevents common health problems such as back pain, neck or shoulder pain and headaches.

Active, athletic women should use a sports bra with a snug fit and extra support when exercising. Activities that involve more bounce (high-impact) put extra stress on the fibrous, supportive breast tissue. Even low-impact activity has an effect, as of course does gravity. Thus, the breasts, regardless of size, require support, and support is best provided through use of a brassiere.