Adapted from source below.
“Breast support comes from the suspensory (Cooper’s) ligaments and connective tissues that pull inward from the inside of the skin of the breast. There is some support gained from the skin on the outside of the breasts, but that skin tends to yield to pressure from the weight of the breasts, and the skin will increase in size accordingly. The ligaments are the major source of natural breast support. Some ligaments and connective tissues are actually formed like a basket (or like a brassiere) inside of the breast, to cradle the interior of the breast and support it. If a woman always wears a bra, the constant, unnatural, external support from the bra will allow the Cooper’s ligaments to atrophy and weaken. If the ligaments and connective tissues are allowed to weaken (from the lack of use), those ligaments and their anchoring points will become sore when they are required to fulfill their designed purpose of supporting the breast when the bra is removed. Women that have never worn a bra or spend adequate time without one on, do not have the pain mentioned above, because their ligaments are stronger. They were allowed to develop at puberty without a training bra, and are up to the task of supporting the breast.”
“In our society today, most breasts are bound in bras, and thus are supported from their earliest signs of development in the “training bras” mentioned earlier, which prevents their ligaments and connective tissues from ever developing beyond a minimum amount (from puberty onward). This means the ligaments are not prepared to do their natural job.”
“Dr. Gregory Heigh of Florida has found that over 90% of women with fibrocystic breast disease find improvement when they stop wearing their brassieres.”