Once upon a time, she was sweet, cuddly, and eager to please. Seemingly overnight, she's become emotional, unpredictable, and sometimes, a downright unreasonable girl. Congratulations—you have a teen (or pre-teen) going through puberty.

First, take comfort in knowing you're not alone. The changes your teen is going through are a completely normal side effect of her development into an adult. There's nothing wrong with her—she's just grappling with the roller coaster that goes along with this enormous life transition. Secondly, remember, with knowledge comes power; so, here's a guide to what's going on behind the makeup, eye rolls, and tears so you know how to help your teen through the process as best you can.

Puberty 101

Puberty marks the life stage when a girl's body changes from a child to an adult. Natural chemicals in the body called hormones help trigger and facilitate this process. These hormones produce changes, some gradual physical changes, and other sudden emotional changes. Although it may seem as if hormones are transforming your teenager into a different person, she's still the same—just the “growing up" version.

Of course, we all went through puberty ourselves, but it can be difficult to remember exactly what to expect as you watch your teen go through it. After all, not all females—even females in the same family—experience puberty in the same way. However, most aspects of puberty follow a fairly predictable pattern.

One of the first changes your teen will notice are growing breasts, usually between the ages of eight and 12. Often, one breast grows faster and larger than the other. This can be distressing, but you can reassure her it's totally normal. Once her breasts start growing, your teen will probably want to buy a bra. Most girls start out with a soft, sports-type bra before moving on to a more substantial style with cups.

How to buy your daughter her first bra.

When breasts begin to look obvious through certain shirts, jiggle, become sensitive, or feel heavy, it's time to consider a bra. The two most important things when it comes to a bra are fit and fabric. A bra is right next to the skin all day long, so if it's a little scratchy or tight when your teen tries it on in the store, it will drive her crazy by the end of the school day.

To determine her bra size, use a tape measure to measure around her chest just under her breasts. If the number is even, add four inches to get her chest size. If she measures 28, for example, add four and look for bras in size 32. If the number is odd, add five to get your size. So, if she's 29 inches, look for bras in size 34. Bras typically come in chest sizes ranging from 28 to 44, with larger sizes available at specialty stores and through catalogs.

Next you need to determine her cup size, or the size of her breasts. Cup sizes range from AAA (very small breasts) to EE (very large breasts). Have your teen measure around her chest, this time lining up the tape with her nipples. If the measurement is the same as the first one, her cup size is AA or AAA. If it's one inch larger, size is A; two inches larger, size B; three inches, size C; four inches, D; five or more, DD or EE.

The measurements will give you and your teen some guidance, but in the real world, bras marked the same size vary considerably, so she may have to try on several sizes and styles to determine which is best; the fit should be smooth and not too tight. Do not buy a bra if it puckers or squeezes her breasts or leaves bulges of flesh poking out around the sides. For women with larger breasts, supportive straps can help prevent back pain. A saleswoman can help you find a bra that fits. Another change your teen going through puberty will experience is hair growth.

Navigating hair growth with your teenage daughter.

Hair will grow on her pubic area, on her legs, and under her arms. Shaving underarms and legs is a personal choice. You may want to remind your teen that some women shave, and some women don't. If she chooses to shave, make sure she uses her own razor, avoids sharing razors with friends, and uses either shaving cream or soap and water as a lubricant.

Shaving: Dos and Don'ts

• Do shave in the direction of hair growth.
• Do change the blade or switch to a new razor often. A dull razor can cut the skin or cause a rash.
• Do be careful when shaving hair in sensitive areas such as the bikini line—shaving without proper lubrication or with a dull razor can cause a rash.
• Do not shave facial hair; shaving facial hair will leave stubble and can make hair grow back dark and thick.
• Do shave at the end of the day when skin is less puffy.
• Do use water and shaving cream or soap because it will help get a close shave without nicks.

Other options (always talk to an adult before trying a new hair removal option):

• Tweezing is an option for isolated hairs. But keep in mind that tweezing can harm hair follicles and make hairs even more stubborn.
• Waxing is a longer-term, somewhat painful option for removal of hair across wide areas, such as legs or hair around the bikini line. Some people have their underarm hair or facial hair waxed. The procedure can leave skin temporarily red and irritated, however.
• Chemical hair removers, such as Nair, are also effective for large areas of the body, but they can leave a rash. Follow label instructions and apply a moisturizer after treatment to reduce irritation.
• Electrolysis uses electricity to destroy the hair follicle and prevent hair from re-growing. The technique, which can be painful, is used primarily for facial hair.
• Laser hair removal, effective for fair-skinned individuals with dark, coarse hair, is an increasingly popular, expensive option. It is less painful than electrolysis and may offer permanent hair removal.

A less noticeable but equally significant change is the widening of your teenager's hips and the slimming of her waist.

Helping your daughter deal with the change in her body shape and potential acne.

Her stomach, bottom and legs might change shape, too. All these changes will make her look more like a woman than a girl, and they are all normal and expected.

Another sign of puberty in some cases is acne. This aggravating condition may be mild (blackheads and whiteheads), moderate (larger inflamed-looking blemishes) or severe (large cysts or nodules). Contrary to popular belief, greasy foods and dirt do not cause acne; acne results from a build-up of oil, microorganisms and dead skin cells in the hair follicles under the skin. When whiteheads rupture (from squeezing, picking or hard scrubbing), the contents spill over and causes inflammation and more acne in the surrounding tissue. To help her combat acne, encourage your teen to keep her face clean. Also, advise her not to squeeze pimples; this can make them much worse and increase the chance of scarring. Often the condition of a teenager's skin during puberty will mimic what his or her parents experienced when they were teenagers. So, if you or your teen's father had acne, she will be more likely to develop it, too. If your teen's s acne is concerning, talk to a health care professional. The condition can be treated.

Want to give your daughter more info? Share these myths and facts with her.

MYTH: Chocolate and oily foods cause acne.

FACT: There is no evidence that these foods cause acne. But acne-prone people should avoid overindulging in chocolate and oily foods because they are typically high in calories and saturated fats and don't provide much nutrition.

MYTH: Repeated face washing will get rid of acne.

FACT: Individuals with oily skin should wash their faces no more than twice a day. Over-washing can dry out the skin, prompting the oil glands to work harder.

If your daughter wants even more tips, here some great ones.

• Wash your face in the morning and at night, and after you work out. You may want to try a cosmetic face mask (usually a combination of moisturizers and other products that help remove dead and dry skin) once a week or a daily benzoyl peroxide product. Benzoyl peroxide comes in strengths ranging from 2.5 percent to 10 percent. Start out with a low strength once a day and use a stronger product if needed.
• Keep your hair off your face and don't squeeze or pick at your pimples. Wash your hands before touching your face.
• Experiment with makeup products; ask a parent, friend or sales clerk for help choosing the right product and for guidance in putting it on. Some makeup products are oil free and may be a better choice if you have acne. While makeup usually can't make a pimple invisible, it can minimize the blemish and give your skin a smoother overall tone.
• Breakouts are loosely associated with stress, so take note of what's going on when your skin erupts. You may find your acne is associated with your period, a challenging test at school or other stress. If you see such a pattern, you may be able to lessen the problem through stress reduction techniques, such as yoga, breathing exercises or just doing something fun.
• Talk to your health care professional or dermatologist if your acne seems worse than average or if it's especially bothersome to you. A variety of prescription treatments are available to combat acne.

And of course the most significant change during puberty for many girls is their first period. Learn more about your daughter's first period here.