Did you know your ears never stop growing? The same goes for our noses. They're the only two body parts that continue to expand as we get older. But while most of us will have knocked growing pains on the head by the time we reach our early 20s, our breasts are another area that tend to fluctuate in size and shape throughout most of our life... How come?

While it’s easy to think that your cup size was predestined, there are actually a lot of things that affect it. Here are the biggest factors that determine the overall size of your breasts.

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 definitely 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, M.D., F.A.C.S., 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, Sherry Ross, M.D., a women’s health expert at Providence Saint John’s Health Center in Santa Monica, California, and author of She-ology: The Definitive Guide to Women’s Intimate Health. Period., tells SELF. That said, you probably won’t see a drastic change if you gain or lose a few pounds. “It usually has to be a significant weight gain or loss to change your breast size,” Dr. Ross says.

3. Your workout routine

If you started lifting recently and noticed your boobs seem a little perkier, that may be related. Doing pectoral exercises 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 bulk up a little, this can cause your boobs to push out a tiny bit more than usual, Albert Matheny, M.S., R.D., C.S.C.S., of SoHo Strength Lab and Promix Nutrition, tells SELF. Keep in mind that these exercises won’t actually increase your breast size—but they might grow the muscle behind the breast, which could make them appear a little bigger.

4. 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 brings about ovulation and stimulates 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 is said to cause swelling (and even a little soreness), according to Hopkins Medicine. This might prompt you to wonder why your boobs have suddenly gotten bigger. 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, according to Hopkins Medicine. Ultimately, your breasts will return to their normal size and texture.

5. Your birth control

Your birth control can do more than prevent an 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, says women’s health expert Jennifer Wider, M.D.. This is because the estrogen and progesterone in hormonal birth control cause something called edema, or water retention, she says—and it’s unlikely to last. “It’s usually most noticeable when someone starts birth control,” Dr. Wider adds.

6. Your pregnancy and postpartum

Pregnancy boobs are a real phenomena—a pregnant person’s breasts can grow several cup sizes during pregnancy thanks to hormonal changes like increases in progesterone, Dr. Khakpour says. Though your breasts already have milk ducts, progesterone helps your body produce more ducts and lobules, which are glands that produce milk.

By the fifth or sixth month of pregnancy, your breasts are fully capable of producing milk, according to Hopkins Medicine. Your breasts may continue to swell during postpartum if you decide to breastfeed, but they typically go back to normal about three to six months after you stop nursing, Dr. Khakpour says.

And if you have a few kids, the effects may be more pronounced. “Some women may experience changes in breast size and shape after multiple births and breastfeeding,” Dr. Khakpour says.

7. Your age

Your boobs probably aren’t the same now as they were when you were 15, and it’s likely they’ll look different down the road. As you approach menopause, hormonal changes have an impact on the size and shape of your breasts. When estrogen levels drop, the connective tissue in the breasts become dehydrated and lose elasticity, according to Hopkins Medicine. Additionally, the breast tissue, which typically prepares to make milk, stops doing so and begins to shrink a bit, Hopkins Medicine explains. 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, there’s often a reason behind it that you can pinpoint. But if you find that you’re experiencing sudden breast changes and you don’t know why, it’s important to talk to your doctor. While it’s likely due to something you haven’t thought of, it could be a sign of a tumor or growth in your breast. Again, don’t panic if you notice changes, but it’s best to get it checked out, just in case, Dr. Wider says.

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