Here’s What Actually Determines Your Breast Size and Shape

Here’s What Actually Determines Your Breast Size and Shape

by Sue Jones
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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.

Is there such a thing as a normal breast size and shape?

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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What determines breast size and shape?

So many things. Let’s dive in:

1. Your family history

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.

2. Your weight

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.

3. Your workout routine 

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.

4. Your period

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. 

5. Your birth control 

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.

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