Some girls can't wait for them to come and look forward to their arrival, but other girls dread them. Most girls have mixed feelings and aren't quite sure why they have them and what to do about them.

What's the mystery subject? We're talking about breasts. If you're a girl, look down and there they are — or will be soon. As girls mature and go through puberty, breasts are one sign that they're leaving girlhood and entering a new stage in life.

When Do Breasts Start Growing?

Breasts can start growing as early as age 8 or as late as 13. Some girls' breasts grow slowly and others grow quickly. Some girls may feel like their breasts will never start growing. But girls start developing at different ages and different rates. One girl might have more developed breasts at 12, while her friend is still waiting.

Breast development happens in stages. The first stage starts during the early part of puberty, when the ovaries enlarge and estrogen, an important female hormone, begins to circulate in the body. Doctors often refer to the early stages of breast development as "breast budding." Get it? Budding — like a flower has buds.

A breast bud is like a small raised bump behind the nipple. After breast budding happens, the nipple and the circle of skin around the nipple (called the areola) get bigger and a little darker. Then the area around the nipple and areola starts to grow into a breast.

As breasts keep growing, they may be pointy for a while before becoming rounder and fuller. For some girls, one breast might be a little bigger than the other one. A girl's breasts may continue to grow during the teen years and even into her early twenties. Fully developed breasts come in all shapes and sizes.

Girls have breasts so they can do an important job later in life. Someday, if they grow up to have a baby, breasts can provide the milk the baby needs. Breast milk is the very best food for babies and mom is the source. And it doesn't matter if those breasts are small, medium, or large. All sizes can make enough milk to feed a baby.

Getting a Bra

Once a girl has breasts, a bra is a good idea, especially when she's exercising and playing sports. Bras can protect breast tissue and keep the breasts supported. Some girls may also like that bras smooth out their silhouettes and make them feel more comfortable. A bra can make a girl feel more comfortable when she's wearing a light shirt, such as a T-shirt.

Like anything new, wearing a bra can take time to adjust to. They can be hard to fasten and adjust. When a bra is on, it can bag or gap, ride up, dig in, or pop open. The straps can slide off your shoulders or dig into them. And a bra can peek out of a girl's clothing.

What About Bra Sizes?

Getting the right size bra can make wearing them easier. For instance, a bra that's the right size won't pinch, gap, or slide around. So it makes sense to spend some time learning about bra sizes.

There are two parts to a bra's size: the chest size (also called the band size) and the cup size. The chest or band size is represented by a number (32, 34, 36, etc.) and is the part of the bra that runs across a girl's chest and around her back. The cups are the parts of the bra that hold the breasts and come in letter sizes (AA, A, B, C, etc.).

Bras come in many sizes, which are different combinations of the chest sizes and cup sizes — 32AA or 34C, for instance. It's important to get both the chest and cup sizes right to ensure a proper fit. Some bra makers now also offer cup options in half sizes to help get a better fit.

You might feel shy about your growing breasts and not know how to find the right bra. Getting measured can help you find your correct bra size. The women who work in the bra departments of stores can help you with this measurement. Or you can can do it at home. Here's how:

• Run a tape measure just under your breasts, all the way around your back and ribcage. The tape measure should rest flat on your skin and lie straight across your back — not so tight that it digs in, but not so loose that it sags down in the back.
• Make a note of your measurement and add 5 inches. That's your chest size.
• To measure cup size, take the tape measure around your body across the fullest part of your breasts.
• Write down this number and subtract your chest measurement from it. The difference between the numbers is a way of figuring out your cup size. If the difference between the two numbers is less than 1 inch, your cup size is AA. If it's 1 inch, your cup size is A; 2 inches, you're a B; 3 inches you're a C, and so on. For instance, if your cup measurement was 33 and your chest measurement was 32, that's a 1-inch difference. Your bra size is 32A.
• If your chest measurement comes out as an odd number (like 31 inches or 33 inches), it's best to round down to the next number. Most bras have a few sets of adjustable hooks and eyes, so you can adjust the tightness. It's a good idea to round down, rather than round up, because bras tend to stretch over time.

What Kind of Bra to Buy?

Many girls' first bra is a sports bra, a type of bra worn by active women of any age. Sports bras prevent breasts from jiggling around when a woman is playing sports or exercising. Because they're flatter in the cup area, sports bras also make good first bras for many girls. In fact, because of sports bras, and dresses and tops that have built-in bras, a girl might not need a more traditional bra early on. Not all sports bras are alike, so try on a few to figure out which ones you prefer.

Besides sports bras, there are a lot of other types of bras. The most natural-looking bra is a soft-cup bra, which doesn't do much to change the shape of the breasts. Soft-cup bras come in different fabrics and thicknesses and some may have an underwire. Underwire is a U-shaped wire inside the fabric that goes under the breasts to help support them. If a girl's breasts are C cup or larger, an underwire bra is a good choice.

Other bras may have more structured cups and some come with padding inside. Minimizer bras are also available for anyone who wants their breasts to appear smaller. These bras are usually made of thicker fabric with bigger backs and straps.

What Else Should I Know?

Some girls may wonder how they can make their breasts grow faster or bigger. There isn't any magic cream or pill that can speed up the process or make breasts larger than they are. In fact, heredity and weight mostly determine breast size. So if a girl's mom has big breasts or small breasts, she can expect to have breasts of similar size. And a girl who has more body fat is more likely to have larger breasts.

Breasts are just one sign that a girl is getting older and on her way to becoming a woman. If you have questions or concerns about breasts or bras, talk to your mom or another older female family member or friend. If you're worried about how your breasts are growing, your doctor is also a good source of information.