Menstruation, or period, is normal vaginal bleeding that occurs as part of a woman's monthly cycle. Every month, your body prepares for pregnancy. If no pregnancy occurs, the uterus, or womb, sheds its lining. The menstrual blood is partly blood and partly tissue from inside the uterus. It passes out of the body through the vagina.

Periods usually start between age 11 and 14 and continue until menopause at about age 51. They usually last from three to five days. Besides bleeding from the vagina, you may have

Abdominal or pelvic cramping pain
Lower back pain
Bloating and sore breasts
Food cravings
Mood swings and irritability
Headache and fatigue
Premenstrual syndrome, or PMS, is a group of symptoms that start before the period. It can include emotional and physical symptoms.

Consult your health care provider if you have big changes in your cycle. They may be signs of other problems that should be treated.

NIH: National Institute of Child Health and Human Development

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All about Menstruation (For Teens) (Nemours Foundation)
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Menstrual Cycle (Department of Health and Human Services, Office on Women's Health)
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Menstrual Cycle: What's Normal, What's Not (Mayo Foundation for Medical Education and Research)
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Menstruation and Menstrual Problems From the National Institutes of Health (Eunice Kennedy Shriver National Institute of Child Health and Human Development)
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What Are Menstrual Irregularities? From the National Institutes of Health (Eunice Kennedy Shriver National Institute of Child Health and Human Development)
Also in Spanish