Dr. Barbara Hays is a board-certified Ob/Gyn in Sacramento, Calif. In our "Ask an Ob/Gyn" series, Dr. Hays answers questions readers have posted on the Healthgrades Facebook page. Q: My periods never come when they're supposed to. Why are they so irregular? A: Periods can vary greatly among women, from the number of days bleeding, to the amount of bleeding, to the length of time between periods. The average, counting from day 1 of bleeding, is 28 days. The range of normal for starting one period to the start of the next period varies up to a week either way; 21 to 35 days is considered normal. Periods that come early, late, stop, or happen more frequently than you expect are considered irregular. If your periods repeatedly occur more than 40 days apart, it's called oligomenorrhea. If they're less than 21 days apart, it's called polymenorrhea. And if your periods have stopped for at least three months, it's known as secondary amenorrhea. A little bleeding between periods is relatively common: spotting between periods, for example, or a little bit of blood when you're ovulating. This small amount of bleeding is rarely a cause for concern. Your menstrual cycle depends on a complex series of actions taking place in your hypothalamus, pituitary gland, ovaries, and endometrium (uterine lining), and changes in any one of these areas could affect your period. The most common reason for a late or missed period is pregnancy. If you have had a negative pregnancy test, or you have no risk of pregnancy, there are many factors that could be causing your irregular period, some benign and some more concerning: Age: Young women who have only recently begun having periods often have irregular ones, and women who are in perimenopause, or about to enter menopause, have irregular cycles, as well. Stress: Stress can suppress the hypothalamus and may cause a missed period. Severe weight loss, eating disorders, or strenuous exercise: Like stress, these factors result in hypothalamic suppression. Medications: Some drugs, including antidepressants and steroids, can cause irregular periods as a side effect. Travel or changes in your schedule, diet or sleep patterns Being sick, usually much more severe than the usual cold or mild flu Going off of hormonal birth control, such as the birth control pill, implant, or progesterone shots Thyroid disorders: Hyperthyroidism can cause less frequent but heaver periods; hypothyroidism causes more frequent, heavier ones. Polycystic ovary syndrome (PCOS): This hormonal condition, which causes enlarged ovaries and collections of cysts, is common among women of reproductive age. PCOS is actually a condition of insulin resistance, which has many health implications besides period problems. Primary ovarian insufficiency (POI): Also known as premature ovarian failure, this term describes the condition in which a woman’s ovaries stop working before age 40. Pelvic inflammatory disease (PID): Left untreated, sexually transmitted diseases can lead to this more serious, widespread infection of a woman’s reproductive organs. Women should always see a doctor whenever they feel like something's “not right”, or causing concern or worry. If your irregular periods are enough of a concern that you're posting this question, see a clinician to help you figure out what the cause is. Especially if you're considering having children, it's better to get examined and evaluated early. Some of these conditions may make it harder to get pregnant, and they require treatment to protect your fertility, as well as preventing problems in the future with your overall health. Have a question for Dr. Hays? Like Healthgrades on Facebook and tell us what topics you’d like to hear more about in future installments.