Irregularity in the menstrual cycle What is the cause? How to know what the cause is? When will periods return to normal? Irregularity in the menstrual cycle Menstrual cycles vary in length from one woman to another. They may occur at the same time each month or be irregular. Typically, a cycle occurs about once a month, but can be as short as 21 days or as long as 35 days and still be considered normal. Menstrual flow lasts about 3 to 7 days. A menstrual period is considered late if it is 5 or more days overdue according to the usual pattern of periods. A period is considered missed if there is no menstrual flow for 6 or more weeks. What is the cause? A late or missed period could be because of any of the following: 1. Pregnancy This is the most common cause of missed periods. If you have had unprotected sex even once in the past several months, see your doctor for a pregnancy test before you consider any of the other possible causes. 2. Stress Stress is the second most common cause of late or missed periods in teenagers. It may be emotional stress or depression. Or it may be physical stress, such as a severe illness, a sexually transmitted infection, rapid weight loss or gain, or strenuous exercise. Dieting or binging and purging may interrupt menstrual cycles. Changes in your usual routine, for example, going on a vacation may also cause your period to be late or missed. 3. Normal development During the first couple of years of menstruation many teenagers have irregular periods. During this time the body's hormones are not yet finely tuned, so the ovaries may not release an egg once every month. As a result, your cycles may be irregular, occurring as close together as 2 weeks or as far apart as 3 months. 4. Hormone imbalance Hormone imbalance is rarely the cause of missed periods. In teenagers, polycystic ovary syndrome is the most common type of hormone imbalance that affects the menstrual cycle. Polycystic ovaries may cause irregular cycles, incr