"Breasts sag for several reasons, including their shape and size, both of which are genetically determined," explains Susan Love, MD, author of several books on breast health and director of the Dr. Susan Love Research Foundation in Santa Monica, Calif.
In addition, pregnancy makes breasts become larger, which can stretch the skin and cause a droopy appearance once the breasts revert to their original size; so can weight gain and loss. As you age, gravity tends to pull breasts down, just as it pulls down the skin.
"Of course, a good-fitting sports bra can help you feel more comfortable," Love adds. It can protect the breasts from bouncing, prevent chafing of the nipples, and minimize just how much your breasts show through your shirt (some sports bras flatten the breasts considerably). But even the tightest, most uplifting sports bra won't keep your breasts from drooping if they’re of a certain size, shape, or age.