Most women have between 11 and 13 menstrual periods each year. You may be different: You may have more or fewer. Missed or irregular periods must be looked at in terms of what is normal for you.Menstrual periods are often irregular during the first few years after menstruation starts. It may take several years for the hormones that control menstruation to reach a balance.Menstrual periods also may be very irregular at the other end of the menstrual years. Many women realize that they are approaching perimenopause and menopause when their otherwise regular periods become irregular. Menopause occurs when it has been 12 months since you have had a menstrual period.Pregnancy is the most common cause of a missed period. If you might be pregnant, treat yourself as if you are pregnant until you know for sure. Use a home pregnancy test as the first step to finding out whether you are pregnant.If you are not pregnant, other causes of missed or irregular periods include:*Excessive weight loss or gain. Although low body weight is a common cause of missed or irregular periods, obesity also can cause menstrual problems. *Eating disorders, such as anorexia or bulimia *Increased exercise. Missed periods are common in endurance athletes. *Emotional stress. *Illness. *Travel. *Medicines such as birth control methods, which may cause lighter, less frequent, more frequent, or skipped periods or no periods at all. *Hormone problems. This may cause a change in the levels of the hormones that the body needs to support menstruation. *Illegal drug use. *Problems with the pelvic organs camera, such as imperforate hymen, polycystic ovary syndrome, or Asherman's syndrome. *Breast-feeding. Many women do not resume regular periods until they have completed breast-feeding.Remember, you can still become pregnant even though you are not menstruating. Practice birth control if you do not wish to become pregnant.Premature ovarian failure is when you stop menstruating before age 40. Surgery, chemotherapy, and radiation therapy to the abdomen or pelvis may cause premature ovarian failure.Other diseases such as irritable bowel syndrome, tuberculosis, liver disease, and diabetes can cause missed or irregular periods, although this is rare. But if any of these diseases are present, you will usually have other symptoms besides menstrual irregularities.If you've skipped a period, try to relax. Restoring your life to emotional and physical balance can help. Many women miss periods now and then. Unless you are pregnant, chances are your cycle will return to normal next month.Check your symptoms to decide if and when you should see a doctor.