Menstruation is a normal, healthy process that a woman's body goes through from her early teen years until menopause. As part of your monthly menstrual cycle, your uterus grows a new lining (the endometrium) to prepare for the possibility of a fertilized egg. If a fertilized egg does not implant in the lining, then the uterus will shed the endometrium; this shedding initiates menstrual bleeding, otherwise known as your period.
The menstrual cycle normally lasts from 21 to 35 days - although teens and women in their 40s may have longer, irregular cycles up to 45 days. If you are nearing menopause, you can expect that the time between your periods will probably get longer, and eventually, stop. Other signs of approaching menopause are hot flashes and mood changes.
If you are you are not approaching menopause and your period becomes irregular it could be a sign of stress, dramatic weight loss, or conversely, sudden weight gain. All of these conditions can affect your body's hormone levels and may cause changes in the length of your cycle. Medications such as antidepressants can also be a common, harmless cause of cycle changes.
Certain endocrine conditions such as polycystic ovaries and thyroid disorders can also cause irregular cycles; these disorders should be checked out by a specialist.
A normal period usually lasts about 3-5 days: anything longer than 7 days is considered prolonged bleeding.