Your Menstrual Cycle Phases—Explained

When it comes to your period, every month seems to look the same: Aunt Flo arrives on relatively the same day, allowing you to stock up on tampons and Midol to weather her visit. And once that week passes, you have three weeks of smooth sailing until you do it all over again. (Sigh.)

But your menstruation cycle is about more than just Mother Nature punishing you for five days and then giving you a free pass to wear white pants and avoid bloating for 23 days. In fact, there's something different happening to your body practically every day of the cycle. And it's generally the same for all women — at least those that aren't on birth control. "When you're taking birth control pills — or any form of hormonal birth control for that matter — it's the BC talking, not your ovaries," says Lauren F. Streicher, M.D., author of Sex Rx: Hormones, Health, and Your Best Sex Ever. (Here are some other reasons your period might be irregular, besides birth control.)

Here's the deal from experts and research to give you a better understanding of how your body works during all 28(ish) days of this cyclical process, offering everything from what color your blood should be to whether you're ready to take a pregnancy test. Check it out.

Days 1 to 3

Your menstrual cycle begins on the first day of your period. "The average menstrual cycle is 28 days, and the normal range is 21 to 35 days," says Sara Gottfried, M.D., author of The Hormone Cure. Therefore, a full cycle is the first day of one period until the first day of your next period, she explains. (Use one of these handy period tracking apps to keep track.)

So why, exactly, are you bleeding? Simply put, if you're not pregnant, your hormones are dropping and the lining of the uterus has reached peak thickness, so it's time to shed.

Periods can be heavy, moderate, or light overall, but these first three days are typically the heaviest. "When menstrual blood is released, the body also releases anticoagulants-chemicals that allow the blood to flow and prevent it from clotting," says Gottfried. "During the heaviest parts of your period, when there is a faster flow, sometimes there is not enough time for the anticoagulants to work, enabling clots to form."

Days 4 to 6

Every woman's flow varies, but you should now be entering the lighter portion of your period. At this point in time, you may begin to notice the color of your blood changing from red to a shade that is closer to brown or black. "The color of period blood all has to deal with how long the blood has been sitting in your vagina," says Streicher. The quicker it's expelled, the closer to bright red it will be. (Here's more on what the color of your period blood means.)

Days 7 to 10

This is the day we all wait for. By day seven of your cycle, under normal circumstances, your period should be nearing its end, says Gottfried. Up next, she explains, is a complex interplay of hormones that cause egg-containing follicles to develop in the ovaries.

Over several days, these little follicles grow and prepare to release an egg. "As the eggs start to develop, they produce estrogen, so your overall estrogen levels will start to rise," says Streicher. This increase in hormones causes the lining of your uterus to thicken in preparation for harboring a fertilized egg. And, eventually, one follicle will be recruited and mature in preparation for ovulation, says Gottfried.

Days 11 to 13

Ovulation takes place in the middle of your cycle, typically day 14, so on day 11, you're just days away from your most fertile span. "The best time to try for a baby is in the days leading up to and the day of ovulation," says Gottfried. If you want to expand your family, now would be a good time to get busy, since sperm can survive for up to five days in a woman's body.

"Estrogen is at its highest levels right before the midpoint of the average menstrual cycle-usually days 12 and 13," says Gottfried. This means your fertility levels are high, your cervical mucous is changing, and the surviving follicle should be ready to release an egg.

Day 14

Your egg is released from the follicle-welcome to ovulation! "Testosterone levels are highest during the midpoint of the menstrual cycle, generally day 14, around the time of ovulation," says Gottfried. Scientists believe testosterone levels rise around this time to increase and stimulate sexual activity in women and increase the likelihood of conception, she explains. (Here are some other weird signs you might be ovulation, including why you likely get horny during ovulation.)

So how long are you fertile? While the exact number of prime days before or after ovulation is uncertain, research published in the New England Journal of Medicine states that estimates range from two days per menstrual cycle to 10 days or more (depending on whether your cycle is longer than the average 28 days). But it is known that you're most fertile the day your egg is released, says Streicher.

Days 15 and 16

Your egg is now on the move-typically taking several days to travel down the fallopian tube. "If the egg is fertilized by sperm on the way down, the fertilized egg will embed in the thickened lining of the uterus and pregnancy will begin," says Gottfried. "If the egg is not fertilized by the time it reaches the end of the fallopian tube, hormone levels change, signaling the next menstrual cycle to begin." (Here are more things ob-gyns wish women knew about their fertility.)

Days 17 to 19

"After ovulation occurs, estrogen levels drop, and progesterone levels start to rise in order to stabilize the growth of the uterine lining and to stimulate menstruation," says Gottfried. Since you're several days removed from ovulation, your progesterone levels are likely skyrocketing. "If embryo implantation doesn't take place, progesterone and estrogen levels will plummet, signaling the body to shed the lining of the uterus and the remnants of the dead egg," she explains.

Curious if you and your partner have conceived? Hold off on a trip to the drugstore-it's still too soon for a pregnancy test to detect anything, says Streicher.

Days 20 to 23

You're now entering PMS territory. "PMS symptoms can start anywhere from one to two weeks before your period starts, and the symptoms usually go away once you start bleeding," says Gottfried. That includes tender breasts, tiredness, bloating, headaches, changes in your appetite, irritability, depression, and acne flare-ups. In fact, 63 percent of acne-prone women experience premenstrual flares, according to a study published in the Archives of Dermatology. The zits begin to surface roughly a week before the onset of your period, and then subside as soon as bleeding begins.

Also worth noting: if you're trying to get pregnant and want to confirm that you've ovulated, this is the perfect time for your doctor to check to see if your progesterone is elevated, says Streicher.

Days 24 to 26

"The second highest peak of estrogen occurs around day 21 or 22 in a 28-day cycle," says Gottfried. So while your progesterone levels will still be up there, around day 24, your estrogen levels will begin to dip again.

Unfortunately, the 25 and 26 days in your cycle tend to be a struggle because PMS is now in full force, explains Streicher-so don't be surprised if feeling bloated is all you can think about. Just remember to stay calm because you're not alone: Research published in the journal Human Reproduction found that it's extremely common for women suffering from PMS to crave high carbohydrate sweets-and the cravings only increase as your symptoms worsen.

Keep in mind, though, that you're probably not eating as much you think you are. Research published in the journal Physiology & Behavior found that during PMS most women only consume roughly 200 calories more per day than they normally would. (Try these holistic PMS treatments to manage your symptoms.)

Days 27 to 28

The good news? Your PMS symptoms should pretty much be gone by now. The bad news is the bleeding starts very soon if you're not pregnant. (Break out the tampons and Midol, again!) "Those surges of estrogen have been thickening the lining of the uterus, so if there is no pregnancy, it's time for the endometrium (which is a fancy term for the top layer of your uterus) to shed," says Streicher.