Puberty usually starts between the age of 8 and 13 and lasts a few years. It may start earlier in girls who are overweight, or later in girls who are very athletic or thin. If you are 12 and haven’t started developing breasts or are 15 and haven’t gotten your period, your doctor will probably want to examine you to make sure everything’s ok.

During puberty, your body releases hormones that stimulate your ovaries to start producing the female hormone estrogen. Gradually, a girl’s body starts changing into a woman’s body. But these surging hormones can also make your mood go up and down — and sometimes it may seem as if your body is out of control.

Growing and Gaining Weight

Most girls experience growth spurts early in puberty, while most boys have them later in puberty. That’s why many girls are taller than boys in middle school.

Breast Development

Along with gaining curvier hips, your breasts grow during puberty. Inside them, a network of milk ducts develops. This is your body’s way of preparing you to nurse a baby when you’re older.

Breast development is one of the changes that stress girls out the most. Many girls worry that their breasts aren’t growing enough. But breasts usually continue to grow until you’re 17 or 18 years old — or even into your 20s. Sometimes one breast grows faster than the other, although the slower one usually catches up.

Menstruation

Two years or so after your breasts start to develop; you’ll probably get your first period. Periods usually last between two and eight days and come every 21 to 35 days. A 28-day cycle is common, but often it takes a while before your periods are regular.

Each month, the lining of your uterus thickens with blood to help a fertilized egg grow. When you don’t become pregnant, that lining sheds, and blood comes out of your vagina. Only a couple of tablespoons of blood are released during your period, but it can seem like a lot more.

Vaginal Discharge

You may notice white, sticky stuff in your underpants. That’s the fluid that keeps your vagina moist and clean. Vaginal discharge may become thicker and stickier at some points during your menstrual cycle. It has a slight odor but most people can’t smell it. Bathing regularly with soap can help reduce the odor if it bothers you.

If your vaginal discharge becomes itchy or irritated, has a strong odor, or is dark yellow or greenish, you may have a vaginal infection.

Body Hair

Puberty brings hair to your body in new places: under your arms, in your genital area, and maybe even on your upper lip. The hair on your arms and legs may also get darker or thicker.

Pubic hair usually starts with a few straight strands and becomes curlier and darker as it grows. Eventually it grows into a thick triangle over the pubic bone and spreads a little to your inner thighs. This growth may start at the beginning of puberty or any time during it.

If you grow hair on your chest or chin, though, see your doctor. You may have a hormone imbalance that needs to be corrected.

Acne

Acne — whiteheads, blackheads, and pimples — is caused by surging hormones.