Menstrual Periods


Menstrual periods are part of a monthly cycle a girl’s body goes through that enables her to reproduce.

Menstrual periods are one of the many changes that take place in a girl’s body during puberty.

Patients can be seen by Texas Children's experts in Pediatric and Adolescent Gynecology.

Causes & Risk Factors

Once a month, hormones cause the lining of the uterus (womb) to grow and thicken with extra blood and tissue in preparation for nourishing a fetus. When an egg is released by the ovaries but doesn’t get fertilized by a sperm cell, the uterus sheds this extra lining it had prepared for a baby. Menstrual bleeding is this lining being shed from the uterus and flowing out of the body through the vagina.

When do girls start having menstrual periods?

On average, girls have their first menstrual period between the ages of 12 and 13. However, like all changes in puberty, every girl’s body changes at a different rate. Some girls may start menstruating as early as age 10, others not until 15. Most girls should have started their menstrual periods by age 16.

As a general rule of thumb, a girl’s first period (known as menarche) usually occurs about two years after her breasts begin to develop. 

Symptoms & Types

Girls often experience physical and emotional symptoms just before or during their periods. These symptoms may include:

  • Cramps
  • Bloating
  • Headache
  • Mood swings and irritability (PMS)
  • Fatigue
  • Acne
  • Food cravings

How long does a menstrual period last?

Menstrual periods typically last 3 to 5 days. However, some girls may have periods that last only a couple of days while others’ may last a week.

What is a menstrual “cycle”?

The menstrual cycle is the amount of time between a girl's periods. To determine this cycle, count from the first day of bleeding in one month, to the first day of bleeding the next month.

Menstrual cycles last an average of 28 days, but may range anywhere from 21 to 45 days. It’s common for girls to have shorter or longer cycles when they first begin having periods. It often takes a few years for the cycle to become regular. 

How much bleeding happens during a menstrual period?

While it may appear to be more, the average amount of blood lost during each menstrual period is only about two tablespoons. The flow will usually be heaviest at the start of a girl’s period.

Tampons or pads are used to absorb the blood. Tampons should be changed every four to six hours to prevent the risk of toxic shock syndrome. Tampons or pads may need to be changed more frequently, especially the first few days of your period.

What causes a missed menstrual period?

There are several common reasons a girl may miss her menstrual period, including:

  • Pregnancy
  • Sudden weight change
  • Illness
  • Stress
  • Extreme exercise
  • Medications
  • Hormone problems

How to know if menstrual periods are normal?

In general, visit with a doctor if:

  • Periods have not started by age 16 (amenorrhea)
  • Periods consistently last longer than a week
  • There is excessive bleeding or periods lasting more than 7 days, using more than 8 pads or tampons per day
  • Periods have not happened for 3 months (amenorrhea)
  • There is bleeding in between periods
  • There are severe cramps/pain before or during your period

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Other Contributors

Jennifer Kurkowski, WHNP