“The majority of women have asymmetric breasts—some can even be off a cup size,” says Mary Jane Minkin, MD, a clinical professor in the Department of Obstetrics, Gynecology and Reproductive Sciences at Yale Medical School in New Haven, CT. “We don’t know the reason that this occurs. But it is highly unlikely to be anything worrisome-it is just a bit aggravating for women being fitted for bras,” Dr. Minkin says.
Rest assured that your asymmetrical breasts are completely natural. But why is one bigger than the other to begin with? Like many other mysterious things about your body, it depends. (Find out how your breasts change as you age.)
Uneven breasts are often inherited, according to Sherry Ross, MD, a women’s health expert at Providence Saint John’s Health Center in Santa Monica. So if your mom and grandmother’s breasts are two different sizes, it’s likely that yours will be, too.
It could also be a matter of weight loss or gain, Dr. Minkin explains. Because your boobs are partially made up of fat (along with connective tissue and milk ducts), their size can change as your weight does. But you don’t always gain or lose weight evenly across your entire body. Same goes for your breasts; one could lose or gain more fat than the other.
Pregnancy and breastfeeding can also impact breast symmetry, Dr. Minkin explains. “Breasts tend to increase in size during pregnancy and breastfeeding and then get smaller after breastfeeding—smaller than they were to begin with in many cases.”
And the baby’s feeding habits can impact a woman’s breast shape as well. “The baby seems to have a distinct preference for one side or the other and that can leave mom with some asymmetry,” she explains. If your breasts have always been symmetric and you develop an asymmetry, then it merits an evaluation and you should get evaluated for a cyst or another abnormality in the larger breast.
What’s more, uneven breasts might be a sign of a condition like scoliosis (a curvature of the spine) or an abnormal chest wall. But don’t worry, genetics and weight change are much more common causes of asymmetry.
You should still pay attention if your breasts suddenly become asymmetrical, especially if there is a mass in one breast that causes the change in size, Dr. Minkin says. “I would counsel a woman to see her healthcare provider for an evaluation and a mammogram and/or ultrasound if necessary—but definitely not to panic.” Watch out for the other signs of breast cancer you might be ignoring.
But again, this shouldn’t be cause for concern. It’s most likely weight gain. All the same, it can’t hurt to get them checked out! And while you’re at it, don’t miss 13 more things your breasts won’t tell you.
A lot of things come in matched sets, but breasts aren't usually one of them. Breast asymmetry—when breasts vary in size, shape, or position—is very common. It's considered normal when even fully developed breasts aren't the same size. The difference is most likely due to normal growth variations that are genetically driven. That means if your mother or grandmother had asymmetrical breasts, chances are that you may, too.
Breast asymmetry is often noticed as early as puberty. In adolescence, when breasts start to develop, one breast may grow more quickly than the other. The reason for this "size differential" isn't clear—it may because girls have more breast tissue cells on one side, or the cells may be more sensitive to the effects of the hormone estrogen, which causes breast tissue cells to grow. Usually, the other breast will start to "bud" shortly after, though it can take a while for it to catch up and even out. In one 2018 study of 300 women requesting breast augmentation with implants, 91% had natural asymmetries.
Medical Conditions Affecting Breast Size
Different conditions can affect the symmetry of your breasts.
Breasts may appear uneven during pregnancy when hormonal changes and the body's preparation for breastfeeding make breasts larger and sometimes lopsided.
During breastfeeding, breasts can vary in size, especially if the baby favors nursing on one side. As long as your child is getting enough breast milk and growing at a steady rate, uneven breasts aren't really anything to worry about.
Atypical Ductal Hyperplasia
Atypical Ductal Hyperplasia (ADH) is a condition where there's an excessive growth of tissue in the breasts' milk ducts. ADH isn't cancer, but it may increase your risk of getting cancer in the future.
In this condition, there are more cells lining the duct than normal, and some of these cells are irregular in shape and size. This can lead to benign lumps in the breast that may affect its appearance.
Hypoplastic breasts, also called underdeveloped breasts, may be small, thin, spaced far apart, or very uneven. The areola may appear very large as well.
Breast hypoplasia often has no identifiable cause, though it may be driven by hormones. It's sometimes due to injury or a medical condition. For instance, there's been one reported case of unilateral breast atrophy following normal breast development from infectious mononucleosis in an adolescent girl.
Juvenile hypertrophy is a rare condition in which one breast grows significantly larger than the other. The cause isn't known, but it's believed to be related to hormone sensitivity or production.
According to an article published in 2017, the condition, which occurs during puberty, often follows a six-month period of extreme breast enlargement, followed by a longer period of slower but sustained breast growth. Juvenile hypertrophy can be corrected with surgery.
When Should I Worry About Uneven Breasts?
Uneven breasts are not usually a cause for concern. That said, research is finding that breast asymmetry may be an independent risk factor for breast cancer.
A number of studies, including one from 2015, have reported that women whose breasts vary in size by over 20% may be at higher risk of developing breast cancer.6 More research is needed to determine the role breast asymmetry plays in breast cancer risk. For now, be aware that if you notice any change in the size, shape, and/or appearance of either breast, you should inform your doctor.
If an underlying condition is suspected, your healthcare provider may recommend a mammogram, a breast ultrasound, or a breast biopsy.
Daily Life and Coping
Asymmetrical breasts are very common and usually nothing to be worried about. At a certain point, however, it can be difficult to hide the difference in breast size: A volume difference approaching 30% or greater is reportedly very difficult to conceal in normal attire.7 A professional bra fitter can help you pick out a bra that fits both sides of your chest or disguises any difference, if this is of concern to you.
If you're uncomfortable about the unevenness of your breasts to the point that affects your daily life, consider meeting with a counselor or mental health professional who can provide support and advise you about how you can learn to accept your body. Joining an online support group about body image or bust size can help you connect with women who deal with similar issues. If you can't accept your breasts or they're causing you health problems, there are surgical procedures available that can help improve the symmetry of your breasts.