Menstruation, or a menstrual period, is a woman's monthly bleeding.  Body chemicals called hormones rise and fall during the month to make the menstrual cycle happen.  Every month, the body prepares for pregnancy. If no pregnancy occurs, the uterus sheds its lining. The menstrual blood is made up of blood and tissue from inside the uterus during a monthly process of shedding. Menstrual blood passes out of the body through the vagina.

Menstrual periods (also called menses) usually start around age 12 when puberty begins (also called menarche) and continue until menopause, which occurs during the early 50's. Periods can, however, begin as early as age 8 or as late as age 16 to fall within normal healthy ranges.

The average menstrual cycle is 28 days from the start of one to the start of the next, but it can range from 21 days to 35 days in adults and from 21 to 45 days in young teens.  For the first few years after menstruation begins, longer cycles are common. A woman's cycle tends to shorten and become more regular with age.

Your period may not be the same every month and it may not be the same as other women's periods. This is normal. Periods can be light, moderate, or heavy, and the length of the period also varies. While most periods last from three to five days, anywhere from two to seven days is normal. Normal menstruation:

• Menstrual cycle = 21 - 45 days for young teenagers
• Menstrual cycle = 21 - 35 days for women
• Longer cycles are common
• Vaginal bleeding lasts from 2-7 days (3-5 days average)
• Periods can be light, moderate or heavy bleeding

Keep reading to understand which symptoms of vaginal bleeding are serious, and when to take action.