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You’ve been in enough locker rooms to know that every woman’s breasts look different. “Almost no one has perfectly symmetrical breasts," says Mary Jane Minkin, M.D., professor of obstetrics and gynecology at Yale School of Medicine. "If they do look exactly like one another, it’s probably thanks to plastic surgery,” she adds.

Still, you’ve probably wondered why your breasts are the way they are. We called up experts to glean a greater understanding behind what determines the shape, size, and feel of your dynamic duo.

Genetics
Far and away, genetics plays the biggest role in the size and shape of your breasts. “Your genes also influence the levels of your hormones, which affect your breast tissue,” says Richard Bleicher, M.D., surgical oncologist and director of the Breast Fellowship Program at Fox Chase Cancer Center in Philadelphia. “Genes determine how dense your breasts are, as well as what your skin is like, which affects the appearance of your breasts.” A study in the journal BMC Medical Genetics analyzed data from more than 16,000 women and found a total of seven genetic factors were significantly associated with breast size. “Your breast characteristics can come from both sides of your family, so genes from your dad’s side can affect what your breasts end up looking like too,” Minkin says.

Your Weight
No matter how big or small your breasts are to begin with, a large proportion of the tissue is made up of fat. So it’s no coincidence that your breasts expand when you do. Similarly, as you lose weight, your breast size could change too. How much fat you lose in your breasts when you drop weight may depend, in part, on the composition of your breasts. Women with dense breast tissue tend to have more tissue and less fatty tissue. If that's you, when you lose weight, you may not notice as significant of a decrease in your breasts as a woman who has a greater proportion of fatty tissue in her breasts to begin with. You can’t feel whether you have dense or fatty breasts (only a mammogram or other imaging would show this), so you may not know which category your breasts fall into. And as for those tiny women with big breasts? Thank genetics!

RELATED: Change Your Bra, Change Your Body

Your Age
Enjoy your perky girls while you can! “Like everything else, gravity takes its toll on the breasts,” Bleicher says. Beneath the surface, your Cooper’s ligaments, delicate bands of tissue, help hold everything up. “They’re not true ligaments like those that hold muscle to bone, they’re fibrous structures in the breast,” Bleicher says. Over time, they can wear out like overstretched rubber bands and become less supportive—eventually causing sagging and drooping. The good news: You can fight back by regularly sporting well-fitting supportive bras in order to reduce the gravitational pull on your Cooper’s ligaments. (Find the best bra for your breast type here.)

Breastfeeding
It’s the blessing and the curse of pregnancy: Your breasts swell to porn-star size while pregnant and nursing, but deflate like a post-birthday party balloon when you wean. It’s not entirely understood why they change so dramatically, but it may be due to fluctuations in hormones and the fact that the skin stretches as the breasts become engorged and may not fully contract to their pre-baby firmness after nursing, Bleicher says.

Exercise
You can do all the chest presses and flies that you like, but they’re unlikely to have any noticeable impact on the appearance of your dynamic duo. “Your breasts sit on top of the pectoral muscles, but aren’t part of them so you can develop stronger muscles underneath your breasts without changing their size or shape,” says Melissa Crosby, M.D., associate professor of plastic surgery at the University of Texas MD Anderson Cancer Center. There are, however, a few exceptions. Bodybuilders and women who participate in fitness competitions often have such low body fat that their breasts appear firmer especially when sitting on top of piles of chest muscle, Crosby says. “There’s some data demonstrating that breast size and density also changes in women who do a significant amount of aerobic activity,” Bleicher says. “This is probably due to the fact that you lose body fat, but your breast tissue components don’t change so you develop denser breasts when you exercise more.”

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