Some girls can't wait for them to come and look forward to their arrival as they would their birthday, first kiss, or a soccer championship.Other girls dread them as much as eating a big plate of Brussels sprouts or changing the kitty litter.

But most girls have mixed feelings and are not quite certain why they have them and what to do about them. Boys don't have them, but they're likely to be curious 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. They are just two mounds of flesh, but they get a lot of attention. As a girl matures and goes through puberty, breasts tell the world that this girl is leaving girlhood and entering a new stage in her life.

Because large breasts get attention (just look at magazines, TV, or movies), some girls may worry if their breasts are small or if they mature more slowly. Girls with large breasts may be unhappy, too, because their shape attracts attention they don't want. The truth is that beauty doesn't come in just one bra size. There are all sorts of female shapes and sizes in the world, and all are beautiful.

But breasts do more than just look nice. Girls have them so they can do an important job later in life. Someday, if the girl grows up to have a baby, her 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 produce enough milk to feed a baby.

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 it's just that girls start developing at different ages and different rates. One girl might have more developed breasts at 12, whereas her friend could be still flat as a board.

Breast development happens in stages. The first stage starts during the early part of puberty, when a girl's 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.

Breast size gets a lot of attention and many girls may wonder how they can make their breasts grow faster or bigger. Despite what some magazine ads say, there isn't any magic cream or pill that can speed up the process or make a girl's breasts larger than they are. In fact, breast size is mostly determined by heredity and a girl's weight. So if a girl's mother has big breasts or small breasts, the girl can expect to have breasts of similar size. And a girl who has more body fat is more likely to have larger breasts.

Getting a Bra

Once a girl has breasts, a bra is a good idea, especially when the girl is 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 less exposed when she's wearing a light shirt, such as a T-shirt.

Some girls look forward to getting their first bras, but others dread that event. Like anything new, wearing a bra can be tough to adjust to. They can be difficult to fasten and adjust. Once a bra is on, it can bag or gap, ride up, dig in, or pop open. The straps can slide off a girl's shoulders or dig into them. And a bra can peek out of a girl's clothing. Not only that, but a girl's brother might think it's just hysterical to pull it so it snaps against her back. (If you are a boy, do not do this!)

The ABCs of Bra Sizes

Wearing the right size bra can decrease the number of other problems a girl will have with her bra. 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, for instance. It's important to get both the chest and cup sizes right to ensure a proper fit. Some bra manufacturers are now starting to offer cup options in half sizes in addition to regular cups to help girls get a better fit.

What Size Bra to Buy?

A girl might feel shy about her growing breasts and not want other people talking about them, looking at them, and measuring them. But it's wise for a girl to get measured so she knows her correct bra size.

The women who work in the bra departments of stores can help a girl with this measurement. A girl also can do it at home if she has her mom or a friend help her. Here's how to do it:

• 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 (such as 31 inches or 33 inches), it's usually a good rule 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, since bras tend to stretch over time.