The key factors influencing breast ptosis over a woman's lifetime are cigarette smoking, her number of pregnancies, higher body mass index, larger bra cup size, and significant weight change.

[2][3] Post-menopausal women or people with collagen deficiencies may experience increased ptosis due to a loss of skin elasticity.

[4] It is also commonly believed that the breast itself offers insufficient support and that wearing a bra prevents sagging, which has not been found to be true.

[5] Plastic surgeons categorize the degree of ptosis by evaluating the position of the nipple relative to the infra-mammary fold, the point at which the underside of the breasts attach to the chest wall.

[6] In addition, when milk production stops (usually as a child is weaned), the voluminous mammary glands diminish in volume, but they still add bulk and firmness to the breast.

Estrogen is also essential to maintaining a fibrous protein called collagen, which makes up much of the breast's connective tissue.

[citation needed] Plastic surgeons describe the degree of breast sagging using a ptosis scale like the modified Regnault ptosis scale below:[8][9] University of Kentucky plastic surgeon Brian Rinker encountered many women in his practice who attributed their sagging breasts to breastfeeding, which was also the usual belief among medical practitioners.

[2] As most women age, breasts naturally yield to gravity and tend to sag and fold over the inframammary crease, the lower attachment point to the chest wall.

Many women mistakenly attribute the changes and their sagging breasts to breastfeeding,[13] and as a result some are reluctant to nurse their infants.

"[2] Also discounted as causes affecting ptosis are weight gain during pregnancy and lack of participation in regular upper body exercise.

[19][21] There is some evidence that bra use reduces the development of Cooper's ligaments, connective tissue that supports breast shape.

That atrophy from bra-wearing may therefore lead to more breast sag in the long run, much as the connective tissue in a limb weakens while it is in a cast and must be re-strengthened afterward.

[22] Studies have actually documented that, after an initial period of adjustment, women experienced a significant increase in comfort and breast firmness from going without bras.

If sagging is present and the woman opts not to undergo mastopexy, implants are typically placed above the muscle, to fill out the breast skin and tissue.