Menstruation, or period, is a woman's monthly bleeding. Every month, your body prepares for pregnancy. If no pregnancy occurs, the uterus sheds its lining. The menstrual blood is partly blood and partly tissue from inside the uterus, or womb. It passes out of the body through the vagina. Periods usually start around age 12 and continue until menopause, at about age 51. Most periods last from three to five days.

You should consult your health care provider if

• You haven't started menstruating by age 16
• Your period suddenly stops
• You bleed excessively, or for more days than usual
• You suddenly feel sick after using tampons
• You bleed between periods
• You have severe pain during your period

Premenstrual syndrome, or PMS, is a group of symptoms that start before the period. It can include emotional and physical symptoms.