While 3-5 days is most common, anywhere between 2 and 7 days is considered normal. See your doctor if your period lasts longer than a week or suddenly changes from what's regular for you.

While you can't easily measure blood loss, most women know when their menstrual flow is heavy. If flow seems excessive, is soaking a pad or tampon every hour for several hours, or you see large clots, tell your doctor.

Missed or delayed periods are common in young women, especially in the early years of menstruation, and things like stress, weight loss, and too much exercise can make periods irregular.

Also, several birth control methods can cause periods to stop and can even be designed for just that purpose. Talk to your doctor if you're normally regular but start skipping periods.

But very intense or lasting pain should be reported to your doctor. Lasting pain in the pelvic area can be a symptom of more serious problems, such as endometriosis -- the growth outside the uterus, usually into the pelvic cavity, of tissue that normally lines the uterus.

Although many of us believe it, research findings on "menstrual synchrony" have varied, and no scientific consensus has been reached.

For most women, periods usually occur from 21 to 35 days apart. In young girls and teenagers, periods are commonly irregular and have cycles ranging between 21 and 45 days. Keep track of your periods on a calendar (which can help you and your doctor determine what pattern is normal for you).

A 2010 study found that women who reported feeling highly stressed were more than twice as likely to report having at least five moderate to severe symptoms with their periods as women with lower stress.

Almost three-quarters of the women who responded disagreed or disagreed strongly.

That's roughly 35 years of periods, with an average of 13 menstrual cycles a year.

But you should see your doctor promptly so he or she can determine the cause. Bleeding between periods can have a number of causes, only some of them serious.

Many women pass clumps of blood, or clots, from time to time, especially on heavy-flow days. If clots are large or if you are soaking through a pad or tampon every hour for two or three hours, call your doctor.

Women can get pregnant at any time during their menstrual cycle, although it is most likely when ovulation occurs, 10-16 days before the next period.

Although some women worry that sex during their period is unhealthy, it's perfectly OK if both partners are healthy and are practicing safe sex in a monogamous relationship.

Blood colors ranging from bright red to deep brown are normal, and different colors can occur over the course of the same period.

Many women find that regular aerobic exercise, such as a half-hour of cycling, brisk walking, or running most days of the week, can ease PMS symptoms. Calcium-rich foods and complex carbohydrates may also help. And experts advise getting enough sleep, avoiding caffeine and alcohol, and eating less fat, salt, and sugar.