5 Things You Didn't Know About Your Period

So you've had your period for a few years -- or for decades -- and you think you're in the know. Then up pops a question from you or one of your friends that no one can answer with certainty.  That doesn't surprise gynecologists, who say they often field menstrual cycle questions from their patients.

Here, three top gynecologists talk about the most common questions they get about periods and what they tell their patients.

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1. Why do I get PMS?

PMS, or premenstrual syndrome, occurs because your body is sensitive to hormonal changes, says Richard P. Frieder, MD, a staff gynecologist at Santa Monica UCLA & Orthopaedic Hospital in Santa Monica, Calif. "In the week or 10 days before your period comes, hormone levels -- progesterone and estrogen -- are changing rapidly," he says.

That can cause symptoms such as bloating, mood swings, headache, breast tenderness, and fatigue in some women, he says.

As many as 90% of women experience some symptoms before their period, according to a study in the Archives of Internal Medicine, but many fewer -- 20% or less -- have symptoms severe enough to interfere with normal activities and relationships and be termed PMS.

Whether you have just a few mild symptoms or full-blown PMS, Frieder advises: "Make your body as healthy as possible. Try to get exercise every day, especially on the day you get PMS. Drink lots of water so you are not dehydrated. Eat every couple of hours. Stay away from alcohol and caffeine."

From there, he believes in treating individual symptoms. If moodiness is a problem, for instance, he sometimes prescribes calcium supplements.

A high intake of calcium and vitamin D seems to reduce the risk of getting PMS, according to a study that followed more than 3,000 women and was published in 2005 in the Archives of Internal Medicine. Women who ate about four servings a day of low-fat milk or dairy foods or fortified orange juice were less likely than those who didn't to develop PMS over the 10-year follow-up.

Some experts have suggested that vitamin D and calcium deficiencies lead to the PMS.

Often, women with more severe premenstrual symptoms report amazing relief when they go on birth control pills, says Frieder, who is also an assistant clinical professor of obstetrics and gynecology at the University of California Los Angeles David Geffen School of Medicine. Low doses of antidepressants are sometimes prescribed to improve the mood swings.

2. Why are my cycles irregular?

Some women don't ovulate regularly and therefore have irregular periods, Frieder says. Stress and illness, for instance, can adversely affect the cycle.

But it's important to know the definition of a "regular" cycle before deciding you are having irregular periods, Frieder says. What some women think is an irregular cycle may not be, he says. A "regular" cycle, Frieder says, means one that is between 25 and 35 days -- counting from the first day of bleeding to the start of your next period.