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All About Menstruation

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Puberty and Periods

Menstruation (a period) is a major stage of puberty in girls. It’s one of the many physical signs that a girl is turning into a woman.

Menstruation can be confusing, just like a lot of the other changes that come with puberty. Some girls can’t wait to start their periods. Others may feel afraid or anxious. Many girls (and most guys!) don’t have a complete understanding of a woman’s reproductive system or what actually happens during the menstrual cycle. That can make the process seem even more mysterious.

Girls usually start to go through puberty between the ages of 8 and 13. Their bodies and minds change in many ways. Hormones kick off changes like growth and breast development. About 2 to 2½ years after a girl’s breasts begin to develop, she usually gets her first menstrual period.

About 6 months or so before getting her first period, a girl might notice an increased amount of clear vaginal discharge. This discharge is common. There’s no need for a girl to worry about discharge unless it has a strong odor or causes itchiness.

When a girl first gets her period, doctors call it menarche. Menarche doesn’t happen until all the parts of a girl’s reproductive system have matured and are working together.

The Female Reproductive System

Baby girls are born with ovaries, fallopian tubes, and a uterus. The two ovaries are oval-shaped and sit on either side of the uterus (womb) in the lowest part of the abdomen called the pelvis. They contain thousands of eggs, or ova. The two fallopian tubes are long and thin. Each fallopian tube stretches from an ovary to the uterus, a pear-shaped organ that sits in the middle of the pelvis. The muscles in a female’s uterus are powerful and are able to expand to allow the uterus to accommodate a growing fetus and then help push the baby out during labor.

When a girl starts puberty, the pituitary gland releases hormones that stimulate the ovaries to produce other hormones called estrogen and progesterone. These hormones have many effects on a girl’s body, including physical maturation, growth, and emotions.

About once a month, a tiny egg leaves one of the ovaries — a process called ovulation — and travels down one of the fallopian tubes toward the uterus. In the days before ovulation, the hormone estrogen stimulates the uterus to build up its lining with extra blood and tissue, making the walls of the uterus thick and cushioned. This happens to prepare the uterus for pregnancy: If the egg is fertilized by a sperm cell, it travels to the uterus and attaches to the cushiony wall of the uterus, where it slowly develops into a baby.

If the egg isn’t fertilized, though — which is the case during most of a woman’s monthly cycles — it doesn’t attach to the wall of the uterus. When this happens, the uterus sheds the extra tissue lining. The blood, tissue, and unfertilized egg leave the uterus, going through the vagina on the way out of the body. This is a menstrual period.

This cycle happens almost every month for several more decades (except, of course, when a female is pregnant) until a woman reaches menopause and no longer releases eggs from her ovaries.

How Often Does a Girl Get Her Period?

Just as some girls begin puberty earlier or later than others, the same applies to periods. Some girls may start menstruating as early as age 10, but others may not get their first period until they are 15 years old.

What's a Cycle?

The amount of time between a girl’s periods is called her menstrual cycle (the cycle is counted from the start of one period to the start of the next). Some girls will find that their menstrual cycle lasts 28 days, whereas others might have a 24-day cycle, a 30-day cycle, or even longer. Following menarche, menstrual cycles last 21-45 days. After a couple of years, cycles shorten to an adult length of 21-34 days.

Irregular periods are common in girls who are just beginning to menstruate. It may take the body a while to sort out all the changes going on, so a girl may have a 28-day cycle for 2 months, then miss a month, for example. Usually, after a year or two, the menstrual cycle will become more regular. Some women continue to have irregular periods into adulthood, though.

As a girl gets older and her periods settle down — or she gets more used to her own unique cycle — she will probably find that she can predict when her period will come. In the meantime, it’s a good idea to keep track of your menstrual cycle with a calendar.

How Long and How Much?

The amount of time that a girl has her period also can vary. Some girls have periods that last just 2 or 3 days. Other girls may have periods that last 7 days. The menstrual flow — meaning how much blood comes out of the vagina — can vary from girl to girl, too.

Some girls may worry that they’re losing too much blood. It can be a shock to see all that blood, but it’s unlikely that a girl will lose too much, unless she has a medical condition like von Willebrand disease. Though it may look like a lot, the average amount of blood is only about 2 tablespoons (30 milliliters) for an entire period. Most girls change pads 3 to 6 times a day. They’ll probably change pads more often when their period is heaviest, usually at the start of the period.

You may be worried about whether your period is normal in other ways. This is normal when a girl first gets her period and isn’t sure what to expect. Your doctor or nurse can answer any questions about your period. Here are some times it’s especially important to talk to a doctor or nurse if:

  • your period lasts longer than a week
  • your blood soaks thorough more than one pad every 1-2 hours
  • you go longer than 3 months between periods
  • you have bleeding in between periods
  • you have an unusual amount of pain before or during your period
  • your periods were regular then became irregular


Lots of girls notice body or mood changes around the time of their periods. Menstrual cramps are pretty common. More than half of all women who get periods say they have cramps during the first few days. Doctors think that cramps are caused by a chemical called prostaglandin that causes the muscles of the uterus to contract.

Cramps can be dull and achy or sharp and intense. Sometimes a woman feels cramps in her back as well as her tummy area. Some girls find their cramps aren’t as bad as they get older. Sometimes they go away completely.

Many girls and women find that over-the-counter pain medicine (like acetaminophen or ibuprofen) can help cramps. So can taking a warm bath or putting a warm heating pad on the lower abdomen. Exercising regularly throughout the monthly cycle may help lessen cramps, too. If these things don’t help, ask your doctor for advice.


PMS and Pimples

Some girls and women find that they feel sad or easily irritated during the few days or week before their periods. Others may get angry more quickly than normal or cry more than usual. Some girls crave certain foods. These types of emotional changes may be the result of premenstrual syndrome (PMS).

PMS is related to changes in the body’s hormones. As hormone levels rise and fall during a woman’s menstrual cycle, they can affect the way she feels, both emotionally and physically. Some girls, in addition to feeling more intense emotions than they usually do, notice physical changes along with their periods — some feel bloated or puffy because of water retention, others notice swollen and sore breasts, and some get headaches.

PMS usually goes away soon after a period begins, but it can come back month after month. Eating right, getting enough sleep, and exercising may help relieve some of the symptoms of PMS. Talk to your doctor if you are concerned about your premenstrual symptoms.

It’s also not uncommon for girls to have an acne flare-up during certain times of their cycle; again, this is due to hormones. Fortunately, the pimples associated with periods tend to become less of a problem as girls get older.

Pads, Tampons, and Liners

Once you begin menstruating, you’ll need to use something to absorb the blood. Most girls use a pad or a tampon. But some use menstrual cups, which a girl inserts into her vagina to catch and hold the blood (instead of absorbing it, like a tampon).

There are so many products out there that it may take some experimenting before you find the one that works best for you. Some girls use only pads (particularly when they first start menstruating), some use only tampons, and some switch around — tampons during the day and pads at night, for example.

Girls who worry about leakage from a tampon often use a pantiliner, too, and some girls use liners alone on very light days of their periods.

Periods shouldn’t get in the way of exercising, having fun, and enjoying life. Girls who are very active, particularly those who enjoy swimming, often find that tampons are the best option during sports.

If you have questions about pads, tampons, or coping with periods, ask a parent, health teacher, school nurse, or older sister.

Reviewed by: Mary L. Gavin, MD
Date reviewed: September 2013