Think you know all there is to know about periods? These period facts will blow your mind (and make you feel really smart the next time you walk down the tampon aisle).
1. A rare period disorder can cause bleeding of the eyes.
Known as vicarious menstruation, this rare (but terrifying) condition makes you bleed from organs besides your uterus—like your eyes—while you're on your period, according to a case study published in the journal Ophthalmic Plastic & Reconstructive Surgery. Luckily, only a handful of cases have been recorded, according to study authors, who say this condition is caused when endometrial tissue (which normally grows in your uterus and sheds during your period) is transmitted through the bloodstream.
2. Most women have period stains in *every* pair of underwear.
54 percent of 36,000 women surveyed by THINX said they ruined every pair of underwear they owned due to their period.
3. Getting your period can worsen asthma symptoms.
In the week leading up to your period, an increased sensitivity to allergens, paired with a lower-than-normal lung capacity, causes between 19 and 40 percent of women with asthma to experience premenstrual asthma (PMA), according to a study published in the journal Multidisciplinary Respiratory Medicine. (The study authors say hormonal changes may be to blame.)
4. Sleeping with a nightlight can help regulate your cycle.
Light exposure affects the secretion of the sleep hormone melatonin, which helps control the release of the female reproductive hormones that determine when your menstrual cycle begins and ends: In a study published in the journal Psychiatry Research back in the early '90s, women with extra long or irregular cycles who slept next to a regular 100-watt light bulb for a few days during the last two weeks of their cycles shortened their cycles by an average of 12 days, meaning this targeted approach could help regulate your periods. The key is to sleep with the light on during the last two weeks of your cycle. Keeping it on at all times might actually make your periods more irregular, since shift workers report irregular periods.
5. There are more than 5,000 euphemisms for periods around the world.
Including: "'Les Anglais ont debarqué," which in French means, "The English have landed," and "Erdbeerwoche," which in German means, "Strawberry week," according to a survey by Clue, a menstrual cycle-tracking app.
6. The sound of your voice changes during your period.
Vocalization researchers theorize that female reproductive hormones actually impact your vocal chords, altering your voice.
7. The average woman has more than 400 periods in her lifetime.
Between the average woman's first cycle and menopause, you can expect some 450 visits from Aunt Flo, according to an article published in the scientific journal The Lancet.
8. There's a reason why tampons are often sold in boxes of 18.
That's the average number of tampons used during each cycle, according to a study published in the journal Contraception, although the same study suggests that some women use up to 48 tampons each month.
9. Before her mission in 1983, NASA engineers asked Sally Ride if she would need 100 tampons for the week of her period in space.
Her savage response: "No. That would not be the right number," according to a 2002 interview published by NASA.
10. You could go through up to 15,000 period products in your lifetime.
During those 450 periods, the average woman uses between 12,000 and 15,000 pads, tampons, and panty liners, as per a Rochester Institute of Technology report.
11. You probably bleed a lot less than you think.
On average, a woman only loses about 60 milliliters, or 2.7 ounces, of blood during each period, according to PubMed Health. (That's almost two shot glasses, or just more than 16 teaspoons.)
12. The "period" you get while taking hormonal birth control isn't a real period.
When you take your white pills (or remove your ring), your body doesn't get the synthetic progesterone needed to keep your uterine lining in place, so it sheds, says Alisa Vitti, creator of MyFloTracker App and author of Woman Code. So while your flow might look the same, it's not caused by the same biological processes as your period, and you lose zero eggs when you're on birth control, says Alyssa Dweck, MD, ob-gyn and co-author of The Complete A to Z for your V.
13. Orgasms can make your cramps feel better.
During your period, uterine contractions caused by an orgasm can release pain-fighting neurotransmitters, like endorphins and oxytocin, Dr. Dweck says. Period sex, FTW!
14. Menstrual cups can safely be worn four hours longer than tampons.
Although most menstruating women find that their menstrual cups are less than halfway full after 12 hours, according to the menstrual-cup manufacturer Diva Cup, the company recommends changing it at least twice a day to prevent infection and triggering toxic shock syndrome (TSS). Meanwhile, tampons have to be swapped out at least every eight hours to prevent related cases of toxic shock syndrome, according to MedlinePlus.
15. American women are less comfortable talking about their periods with guys than women abroad.
Only 35 percent of U.S. women reported no discomfort in discussing their periods with male coworkers and classmates, according to a survey by female health app Clue. On the other hand, 50 percent of Spanish women said they were cool talking about their flows with male colleagues and classmates. Nearly 50 percent of women surveyed in Argentina and the Philippines said the same.
16. Disney made a movie about periods in the 1940s.
The magical Disney production you've never seen is called “The Story of Menstruation” and was funded by Kotex. Essentially, it’s an explainer for tweens on what’s about to hit their ovaries very soon.
17. Women paid for the first disposable pads by putting money in a box on the store counter.
1921, the first disposable pads in the U.S. were sold in a plainly wrapped, unidentified box to keep period product purchasing on the DL, according to Kimberly-Clark, Kotex's parent company.
18. The first tampon patent was filed by a dude in 1931.
The original tampon with a cardboard applicator was patented more than a decade after disposable pads made their secretive store debut, according to the United States Patent and Trademark Office. You can thank Earle C. Haas from Denver, Colorado for his contribution to your cycle.
19. Your natural musk changes while you're on your period.
Men's testosterone levels rise when they whiff clothing worn by an ovulating woman, but not a menstruating one—a sign they're more aroused by women who are ripe for baby making (and off their periods), according to a 2009 study from Florida State University. (The scent change—which is diminished by hormonal birth controls—is otherwise undetectable, since its caused by odorless steroids.)
20. You might spend more money during your period.
When researchers from the U.K.'s University of Hertfordshire surveyed more than 400 menstruating women on their spending habits during different parts of their cycle, they found that women spent more money and shopped more impulsively while menstruating than during other phases of their cycle.
21. Your period probably isn't syncing up with your roommates' or friends'.
When researchers monitored the cycles of 186 women living in the same dorm for more than a year, they found that the women’s periods didn’t sync up after all, according to a 2005 study published in the journal Human Nature.
22. Water can temporarily cork your flow.
Although your uterine lining continues to shed when you're fully submerged in water, the pressure from the water you're in counteracts the gravity of blood exiting your vagina, says Jessica Shepherd, MD, ob-gyn and U by Kotex partner. "It's not that you stop bleeding, it's more that there's pressure to stop blood from flowing out." The more you know...until you get out of the pool.
This article originally appeared on Cosmopolitan.com. Minor edits have been made by the Cosmo.ph editors.