You’ve been getting your period since you were 13, and you still can’t say for certain if it’s in your best interest to swim in the ocean or if it’s possible to get pregnant during your cycle. (Yes, people have period sex.) There are the obviously wrong ideas, like every woman’s cycle lasts the same amount of time (it doesn’t), and a heavy flow indicates menorrhagia (very few women experience this abnormal level of bleeding).

Some ideas, however, are not as easy to disprove. In which case, we poked around the Internet for the wildly perpetuated ideas regarding a woman’s menstrual cycle and ran them by Dr. Carolyn Dean, on the medical advisory board for the Nutritional Magnesium Association.

Myth: Swimming In The Ocean On Your Period Attracts Sharks

Even if Dean wasn’t a doctor specializing in women’s health, she lives in Maui; she knows about sharks. “Sharks don’t just swim around looking for victims,” she said. “They can sense body fluid and blood from distances, but they’re looking for dead fish. It’s possible they sense blood, but they don’t attack. To say you shouldn’t go swimming because you have your period is really ridiculous.”

Myth: Hot Water Increases Period Flow

Hot water (and heat in general) does not affect flow, making it lighter or heavier, but it will increase your circulation, Dean said. Improved circulation benefits women suffering from cramps. For when cramps are especially bad, Dean recommends warm, Epsom salt baths.

Myth: You Can’t Get Pregnant On Your Period

“That’s a mistake; It can happen,” Dean said. “There are other things that can trigger hormones to trigger ovulation.”

For example, Dean said sperm can survive 48 hours after the fact. So if women have sex in the days after their period ends, they may not be safe from pregnancy. The only way you can tell is if you conduct the standard ovulation method, which is to check mucus and temperature. Dean said mucus indicates acceptance of sperm, and temperature indicates ovulation.

Myth: Avoid Exercise

Exercise, like hot water, actually benefits period cramps, producing chemicals that block pain. Dean said exercise can be particularly helpful for people who otherwise enjoy it.

According to Everyday Health, the only time women on their period should avoid exercise if they’re experiencing severe cramping (dysmenorrhea) or menorrhagia.

Myth: Premenstrual Syndrome (PMS) Is Not Real

As soon as we ask this question, Dean laughs. “We don’t want it to be real, but mineral deficiencies and a hormonal imbalance causes what we know as PMS,” she said. “It’s not psychological, it’s physiological.” PMS symptoms vary among women, but they generally include breast tenderness, bloating, fatigue, irritability, and mood swings.

To help, Dean encourages her patients to take magnesium supplements in the weeks leading up to their period. The University of Maryland Medical Center agrees, reporting 360 milligrams of magnesium daily for three days on the day before menstruation starts reduces cramps and menstrual pain.

Myth: Women Don’t Need Their Period

There is a market for oral contraceptives that prevent ovulation, which means women only get their period a certain amount of times a year versus every month. An example is a drug called Seasonale, and while women still have to take a pill every day, their period will only occur every 12 weeks. “I think that’s horrendous, and it could contribute to infertility,” Dean said.

Women who eventually hope to get pregnant want their period, as annoying as the cramps and incessant chocolate cravings are. An irregular cycle (and hormones) can make it harder to conceive, possibly leading to additional fertility drugs and their respective side effects.

Are there any other ideas and myths you were hoping we’d address about menstrual cycles? Check out the comprehensive infographic below.

Menstruation and the Menstrual Cycle (Infographic)