What Determines Breast Size and Breast Shape?

Most body parts grow to a certain size and then stop. When it comes to your breast size? Not so much. Your breast size and shape can change quite a bit throughout your life. (If you’re someone whose boobs get bigger around your period, you know what we’re talking about.)

Believe it or not, your boobs started growing in the womb with the development of your milk-duct system. Then, during puberty, they change thanks to increasing estrogen levels. (Estrogen is a hormone that builds fat in your connective tissue.) Your breast size, areola, and nipple size generally grow larger, making changes in breast shape and size more noticeable. Your boobs will likely continue to change throughout your life, which is totally normal.

If you’re wondering why your breasts appear a certain way, there is not just one answer. How your boobs look can depend on several factors, from your biology to your age and workout routine.

Ahead, everything you need to know about your boobs, including changes you might see over time.

Nope. “Breast size and shape vary so much that it’s hard to pin down a certain archetype,” Marco Harmaty, MD, a plastic and cosmetic surgeon and associate professor at Mount Sinai, tells SELF. You may have heard that there are different breast shapes, such as “round,” or “teardrop” but the truth is, there is no authoritative guide to breast shapes, notes Dr. Harmaty. However, there is one characteristic that many breasts share: Being asymmetrical, meaning one boob looks different than the other. For instance, your left breast may look rounder and larger compared to your right breast. So if one of your breasts looks different from the other, know that this is extremely common and you’re not alone.

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So many things. Let’s dive in:

Just as your genes help dictate your hair and skin color, how tall you are, and a bunch of other characteristics, they also have some impact on your breast size. That doesn’t mean you’re guaranteed to be a C-cup if other people in your immediate family are, but it’s more likely for you than someone who comes from a family with a history of A-cups.

“Women often are born with their breast size, but it can change in their lifetime,” Nazanin Khakpour, MD, FACS, a surgical oncologist specializing in breast cancer at Moffitt Cancer Center, tells SELF. In short, family history is one indicator of many, and there are other factors that contribute to the fluctuations that can happen over time.

Your breasts are a complex part of your anatomy, made up of supportive or connective tissue, milk glands and ducts, and fatty tissue. How much of each tissue type you have is unique to you. Some people have more supportive tissue than fat and vice-versa. If your breasts contain a higher concentration of fatty tissue, you could see a difference in your boob size when you gain or lose weight, Sheryl Ross, MD, FACOG, ob-gyn at Providence Saint John’s Health Center in Santa Monica, California, tells SELF. Keep in mind that weight loss can look different for everyone. Some people may notice their boobs are smaller or shaped differently after losing weight, while others may not. But in general, losing large amounts of weight (think: over a hundred pounds) can lead to smaller breast size and possibly drooping¹.

If you started lifting recently and noticed your boobs seem a little perkier, there could be a connection there. Doing pectoral exercises, such as the chest press, can strengthen your pecs, which are four major muscles that sit behind your breast tissue and facilitate deep breathing and arm movement. If your pecs muscles grow, this may cause your boobs to push out a tiny bit more than usual, Albert Matheny, RD, CSCS, of SoHo Strength Lab, tells SELF. Keep in mind that these exercises won’t actually increase your breast size—but they might grow the muscles behind your breasts. If those muscles become larger and push your breast tissue out further than before, then your boobs may appear a little larger.

Ever wonder why your boobs get bigger around your period? Your menstrual cycle can bring pretty distinct changes to your breast size, texture, and shape. During the first half of your cycle, your body produces estrogen, a hormone that stimulates ovulation and the milk ducts in the breasts, Hopkins Medicine explains. But in the second half of the cycle (as you get closer to your period), progesterone stimulates the formation of milk glands, which can make your breasts swell, according to Hopkins Medicine. So your boobs may temporarily feel a little bigger because of the swelling. While you’re on your period, your breasts might also feel a bit lumpier than usual, but this isn’t a cause for concern—your glands are simply enlarging to prepare for a possible pregnancy. Ultimately, your breasts will return to their normal size and texture. 

Your birth control can do more than prevent unintended pregnancy and help regulate your period: Hormonal birth control methods like the pill, the shot, and the hormonal IUD can actually impact your breast size, Jennifer Wider, MD, tells SELF. This is because the estrogen and progesterone in hormonal birth control can cause water retention, she says—but it’s unlikely to last. “It’s usually most noticeable when someone starts birth control,” Dr. Wider adds.

That said, research about weight gain and changes in breast size is conflicting. In some studies, people report gaining weight while taking birth control, but other studies show that some individuals lose weight while taking birth control². It’s not clear why this happens. Experts theorize birth control containing the hormone progesterone³ may increase some people’s appetite, possibly leading to weight gain and changes in breast size, according to Teresa Tan, MD, an ob-gyn with Stanford Children’s Health. However, no studies have found a significant link between weight gain and birth control.

Pregnancy boobs are a real phenomenon—and breast changes, such as tenderness and swelling, are some of the earliest signs of pregnancy. A person’s breasts can grow several cup sizes during pregnancy due to a few factors, including normal weight gain, water retention, and hormonal changes⁴, Dr. Khakpour says. During pregnancy, your progesterone levels increase to help your body produce more milk ducts and lobules, which are glands that produce milk. As a result, your breasts may become larger, and your nipple size may change and appear darker⁵. Your breasts can continue to swell during postpartum if you decide to breastfeed, but they typically return to your normal size about three to six months after you stop nursing, Dr. Khakpour says. 

Bodies continuously change, so your boobs may look different in the future compared to how they appear now. As you approach menopause, hormonal changes can impact the size and shape of your breasts. When estrogen levels drop, the connective tissue in the breasts becomes dehydrated and loses elasticity, according to Hopkins Medicine. Additionally, the breast tissue shrinks and loses some of its shape. Most people’s breasts will become less perky with time, and that’s totally normal, Dr. Ross says. “It’s largely due to a change in skin elasticity and stretched ligaments,” she adds.

While it’s normal for your boobs to change over time, it’s important to talk to your doctor if you experience sudden breast changes and you’re not sure why. For example, if you suddenly have breast pain, notice a firm spot on your breast, or experience nipple discharge, those are good reasons to make an appointment, as they could be a sign of a tumor or growth in your breast.

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First of all, there is no ideal boob size or shape that anyone should feel pressured to emulate. And it’s important to be aware of gimmicks that tell you otherwise. For example, some supplements claim to “enhance breasts, but science doesn’t back up such claims. Many of these supplements contain herbs such as saw palmetto, which doesn’t do much in terms of growing boobs, according to the Mayo Clinic. What’s more, supplements aren’t regulated, so there’s no way of knowing the ingredients a particular product contains⁶. Further, certain supplements may cause other drugs to be less effective or lead to side effects, such as blood thinning, according to the U.S. Department of Health and Human Services. The bottom line? It’s probably best to steer clear of supplements that claim to enhance your breasts; if you’re intent on trying one, get the OK from your doctor first.

At the end of the day, breast augmentation is the most effective way to permanently change your breast size. The procedure involves placing implants filled with saline or silicone behind the breast tissue or under the chest muscle, according to the National Library of Medicine. Opting for breast implants is a highly personal decision, and it’s important to talk to a surgeon you trust about safety and potential risks. You can search for board-certified plastic surgeons (who are proven experts in their field) on the American Board of Plastic Surgery website.

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If you’ve ever gotten frustrated while bra shopping, you know that bra cups and sizes are not universal. Because of this, there isn’t a standard method to measure breast size. “Cup size is a very subjective measurement. It can vary greatly between different companies,” Dr. Harmaty says. “The measurements you get from a commercial manufacturer may be different from a custom bra company.” Plastic surgeons typically estimate the volume of breasts using a 3D camera and computer. But that measurement is in grams and doesn’t translate to cup size. Dr. Harmaty uses the 3D image and a person’s history of cup sizes to give people personal estimates.

If you really want to know how to measure breast size when you’re buying bras, head to the website of the brand you’re interested in. Many retailers offer directions to help you measure band and bust size, along with a corresponding size chart for their products. Pro tip: You’ll need a soft tape measure. 

The bottom line: Breast size and shape are dependent on numerous factors, many of which are outside of your control. All boobs are different, and there is absolutely nothing wrong with that.

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Sources:

1. Journal of Cutaneous and Aesthetic Surgery, Mastoplasty After Massive Weight Loss: Redefinition and Stabilization of the Breast Mound with Submuscular Autoprosthesis
2. Institute for Quality Efficiency in Health Care, Contraception: Do Hormonal Contraceptives Cause Weight Gain?
3. Maturitas, Sex Hormones, Appetite and Eating Behavior in Women
4. Institute for Quality Efficiency in Health Care, Pregnancy and Birth: Weight Gain in Pregnancy
5. International Journal of Women’s Dermatology, Physiologic Changes of Pregnancy: A Review of the Literature
6. StatPearls, Herbal Supplements

Related:

• 37 of the Best Bras for Lounging Comfortably at Home
• What You Need to Know Before Using Natural Cycles, the First FDA-Approved Birth Control App
• 18 Reasons You Have Itchy Breasts or Nipples—and How to Find Relief