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menstruation 101

a quick refresher course to teach you all about your menstrual cycle.

Medically reviewed by Dr. Jennifer Conti, MD

what is a period?

Here at Thinx—as you can imagine—we spend a lot of time talking about menstruation. As the first menstrual product innovation in almost a century, we designed reusable period underwear as a more eco-friendly way to transform your cycle — with styles that absorb up to 5 regular tampons’ (and 2.5 regular pads’) worth of period flow. But what *is* menstruation actually? For many of us who have a handle on basic period knowledge, the nitty-gritty details may not be super clear. That’s why we put together a quick explainer on what menstruation is, why it happens, and a breakdown of all the phases of your menstrual cycle.

what actually happens during menstruation?

While most of us received a rudimentary education about periods in grade school (which probably involved first kicking all the boys out of the room), it’s possible the specifics have become a bit fuzzy since then. Here’s the thing — on average, you’ll menstruate for 3,000 days in your lifetime. That’s a lot of time to be kind of hazy on the details! So, let’s take this opportunity to go back to the basics: What’s actually going on down there all month?

In a nutshell, if you menstruate, your body is always preparing for one of its eggs to be fertilized. The lining of your uterus will thicken around one of your eggs, creating a cozy home in preparation for a fertilized egg. If you don’t become pregnant, your body sheds the lining and releases the unfertilized egg. Then, the cycle starts all over again! Let’s break that down.


Day 1 of your period is also day 1 of your whole menstrual cycle. This is when your body gets rid of unneeded stuff: the uterine lining, the unfertilized egg, and mucus. And of course, blood flows out your uterus through your vagina and into your pad, cup, or tampon — or into your Thinx underwear, if you’re free bleeding! As you’re probably aware, menstruation tends to last 3 to 7 days and is accompanied by all sorts of fun things like cramping, mood swings, fatigue, and more.

follicular phase

Did you know we’re born with all the eggs we’ll ever have? We usually start with 1 to 2 million follicles (immature eggs), but by the time puberty comes and menstruation begins, the number has dropped to around 400,000. These immature eggs are stored in our ovaries, each inside its own microscopic, fluid-filled follicle. The follicular phase begins when 1 of about 20 competing follicles matures into an egg and ends with ovulation of 1 egg. The first day of your period coincides with your follicular phase, so you’re technically double-booked with menstruation for the first week.

Let’s talk hormones: In our brains, we have a gland called the hypothalamus, which is responsible for things like thirst, hunger, sleep, sex drive, and hormones. At the beginning of your cycle, the hypothalamus kicks things into gear by telling its buddy, your pituitary gland, to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). That’s when your follicle matures and starts preparing for ovulation, the next phase of menstruation. These multitasking follicles then release another hormone, estrogen, which causes the lining of your uterus to thicken in preparation for the fertilization of your egg.


Once your estrogen levels hit their highest point, your hypothalamus gets another message: Release a burst of LH. This surge causes one follicle to burst open and release an egg, which then travels into your fallopian tube. Ovulation typically lasts 1 day, usually around the 14-day mark of your cycle. Tracking when your period starts and ends can give you a better sense of when you ovulate. This is especially useful if you’re trying to get pregnant, or if you’re not using hormonal birth control and trying to avoid having sex when you’re fertile.

If you’re using hormonal birth control like the pill, you usually won’t ovulate because the constant level of synthetic hormones prevents the peak that kicks off ovulation. The bleeding you may experience every month when you take a pill break is actually not a period at all; it’s withdrawal bleeding that’s caused by your body taking a break from those hormones.

luteal phase

The luteal phase is when premenstrual syndrome (PMS) kicks in, with mood swings, bloating, fatigue, cravings — you know the drill. The follicle that popped that egg out then transforms into the corpus luteum. The corpus luteum releases progesterone that stabilizes your thickened uterine wall in anticipation of you becoming pregnant. Here’s where the road forks: If your egg was fertilized, your body will need its uterine lining to remain thick, so the embryo will begin to produce human chorionic gonadotropin (hCG), the hormone that shows up on a positive pregnancy test. But if your body is sure there’s no potential fetus, your estrogen and progesterone levels will decrease, and your uterine lining will begin to disintegrate, which will lead to the beginning of your period.

Now we’re back to square one, day 1 of your period, and you get to start the cycle all over again. (Yay!/?) The good news is that the more familiar you get with what’s going on with your body, the better you can prepare for each phase.