The menstrual cycle is not the same for every woman. On average, menstrual flow occurs every 28 days (with most women having cycles between 24 and 34 days), and lasts about 4-7 days.

However, there is a wide variation in timing and duration that is still considered normal, especially if your periods began within the last few years.

A small percentage of women have periods more often than every 21 days or less often than every 35 days. These variations may be normal.

Some examples of abnormal bleeding include:

• Bleeding or spotting between periods
• Bleeding after sex
• Bleeding more heavily (passing large clots, needing to change protection during the night, soaking through a sanitary pad or tampon every hour for 2 - 3 hours in a row)
• Bleeding for more days than normal or for more than 7 days
• Menstrual cycle less than 28 days (more common) or more than 35 days apart
• Bleeding after you have gone through menopause

Often, if you are bleeding from the rectum or there is blood in your urine, you may think the blood is coming from the vagina. To know for certain, insert a tampon into the vagina to confirm that it's the source of your bleeding.

A change in hormone levels is a common cause of abnormal menstrual bleeding. This is called dysfunctional uterine bleeding.

Other causes of abnormal menstrual bleeding include:

• Endometrial hyperplasia (thickening/build-up of the lining of the uterus)
• Cancer of the uterus
• Uterine fibroids, uterine polyps (small noncancerous growths in the lining of the uterus), adenomyosis
• Medical conditions such as thyroid and pituitary disorders, diabetes, cirrhosis of the liver, and systemic lupus erythematosus
• Pregnancy complications -- such as miscarriage or ectopic pregnancy (when a fertilized egg implants somewhere other than the uterus)
• Changes in birth control pills or estrogens that you take
• Use of certain drugs such as steroids or blood thinners (for example, warfarin or Coumadin)
• Use of an intrauterine device (IUD) for birth control
• Recent trauma, surgery, or other uterine procedure
• Infection in the uterus (pelvic inflammatory disease)
• Bleeding disorders such as Von Willebrand disease
• Polycystic ovary syndrome
• Stress, change in diet or exercise routine, recent weight loss or weight gain, travel, or illness