If you're a woman who grew up playing with Barbie dolls, you may have spotted a difference between the fashion doll's pert chest and the lower-hanging breasts of mothers, grandmothers, teachers, and other women you saw around you. And as you got older, you probably couldn't escape media images of what a woman should look like, including how her breasts should be shaped. Now that you're an adult, you might notice your own breasts losing the fight with gravity.

Rest assured that breast sagging - also called breast ptosis - is natural for an aging, changing body. Here are some of the reasons why it happens.

Aging: Because of the way that breasts are built, sagging is common and largely unavoidable. After all, breasts contain no muscles of their own to hold them up. Rather, breasts are made of fat, glands, milk ducts, and a type of connective tissue called the "Cooper's ligaments," which are thought to provide some structural support. As a woman gets older, these ligaments may get stretched and lose elasticity. Breasts may lose fullness and size as they lose their underlying support system of tissue and fat.

Running without proper support: When a woman runs, jogs, or engages in other vigorous exercise, her breasts move in three directions - vertically, horizontally, and in an overall figure-8 like motion. Without a properly fitted sports bra, breasts move too freely and may stretch the Cooper's ligaments and lead to sagging.

Changes in weight and/or hormones: This includes fluctuations due to weight gain and loss due to dietary habits as well as pregnancy. The more pregnancies a woman has, the more likely her breasts are to shrink and lose their lift.

Smoking: You knew that smoking could harm your lungs and your heart, but did you know that it could change the shape of your breasts? Smoking destroys elastin, the protein that gives skin its elasticity. With lower levels of elastin, the breasts droop and sag.

One thing that doesn't cause breasts to sag is breast-feeding. Research has shown that it is simply the size changes that a woman's breasts undergo through pregnancy that make the difference - not whether she breast-feeds her child. Breast milk is still the best food for a growing infant, so do not let worries about sagging breasts deter you from breast-feeding.

Breast sagging may be reduced by wearing a properly-fitted, supportive brassiere, especially when working out. For women who have stretched skin and decreased breast volume, a breast lift may be an option. But women who are pregnant, breast-feeding, or planning to lose weight should not consider the procedure, as breast shape may change.

Amy Toffelmire