You’ve spent enough time in locker rooms to realize that everyone’s bosoms appear different. “Hardly anyone has perfectly symmetrical breasts,” states Mary Jane Minkin, M.D., a professor specializing in obstetrics, gynecology, and reproductive sciences at Yale School of Medicine. “If they do look identical, it’s probably due to cosmetic surgery,” she adds.

Nonetheless, you’ve likely pondered why your breasts have their unique qualities and what precisely determines their size and shape. Here, experts elucidate the factors that contribute to the shape, size, and sensation of your dynamic pair.

Your Genetic Makeup

Unquestionably, genetics play the most significant role in determining breast size and shape. “Your genetic makeup also impacts the hormone levels in your body, which in turn affect your breast tissue,” explains Richard Bleicher, M.D., a surgical oncologist and the director of the Breast Fellowship Program at Fox Chase Cancer Center in Philadelphia. “Genes determine the density of your breasts and the nature of your skin, both of which influence the appearance of your breasts,” he clarifies. Research supports this notion: a study published in the journal BMC Medical Genetics examined data from over 16,000 women and discovered seven genetic factors that were significantly associated with breast size.

Furthermore, these genetic influences are not solely derived from the maternal side. “Your breast characteristics can arise from both sides of your family, which means your father’s genes can impact the final appearance of your breasts as well,” adds Dr. Minkin.

Body Weight

Regardless of the initial size of your breasts, a substantial portion of the tissue is composed of fat. Hence, it is no coincidence that your breast size may increase if you gain weight. Likewise, losing weight can also lead to changes in your breast size.

The extent to which your breasts reduce in size when you shed pounds may depend, in part, on the composition of your breast tissue. Individuals with dense breast tissue often possess more tissue and less fatty tissue. Consequently, if you fall into this category, the decrease in breast size may not be as prominent compared to someone with a higher proportion of fatty tissue. Since you cannot discern whether your breasts are dense or fatty (only a mammogram or other imaging techniques can demonstrate this), you may not know the exact classification of your breasts.

Aging

It is indeed true that breasts tend to sag as one grows older. “Like everything else, gravity has its effects on the breasts,” emphasizes Dr. Minkin.

. Beneath the surface, fragile strips of tissue known as Cooper’s ligaments assist in holding everything up. “They’re not genuine ligaments like those that keep muscle to bone; they’re fibrous structures in the breast,” clarifies Dr. Bleicher. Over time, they can deteriorate like overstretched elastic bands and become less supportive — ultimately leading to sagging and drooping. The great news is that you can combat this by regularly donning well-fitting supportive bras to decrease the gravitational force on your Cooper’s ligaments.

Nursing

IYKYK: Your bosoms change during pregnancy and even afterwards. Specifically, they enlarge while pregnant and breastfeeding, but deflate like a post-celebration balloon when you wean. It’s not completely understood why they undergo such significant transformations, 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, explains Dr. Bleicher.

Physical Activity

You can perform all the pectoral muscle presses and flys that you prefer, but they’re unlikely to have any noticeable impact on the appearance of your dynamic duo. “Your bosoms rest on top of the pectoral muscles but aren’t part of them, so you can develop stronger muscles underneath your bosoms without altering their size or shape,” says Melissa Crosby, M.D., a board-certified plastic surgeon specializing in reconstructive surgery for patients with cancer.

There are, however, a few exceptions. Bodybuilders and those who participate in fitness competitions often have such low body fat that their bosoms appear firmer, especially when sitting on top of piles of chest muscle, notes Dr. Crosby. “There’s some data demonstrating that bosom size and density also changes in women who engage in a significant amount of cardiovascular activity,” adds Dr. Bleicher. “This is probably due to the fact that you lose body fat, but your bosom tissue components don’t change so you develop denser bosoms when you exercise more,” he explains.

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