Disclaimer before I write anything science-y: I'm not a trained medical professional and I have no scientific background in understanding female health and fertility. Rather, I share learnings from research I've done, experiences I've had, and science I have read and will aim to cite all relevant sources.
After spending 10+ years taking various forms of hormonal birth control (HBC), including the pill, the ring, and the patch, I decided to "quit" putting synthetic hormones into my body every day and recover my natural cycle in 2017. (You can read my full birth control saga here, if you're so inclined.)
Still, as investigative as my decade-long birth control experience was, I never learned much from my doctors about the specific functioning of HBC. Because I bled every month and my cycles were 28 days, I assumed birth control was a kind of bio-hack - nothing scary. And in essence, I wasn't wrong. HBC is a bio-hack, in that it takes our natural menstrual cycle, tears it in half, throws away one half, and continuously replays the other, month after month, until you tell it to stop. But this cycle alteration requires synthetic hormone intervention. And two-years after I ditched HBC, I finally learned how HBC actually works to alter every cell in your body. More on that later.
For now, I want to talk about birth control fundamentals.
Below, I've laid out the super-bare-bones basics of your natural cycle vs. your cycle on birth control. Notice how your HBC cycle cuts out the first half (follicular phase) of your cycle entirely? And how it also cuts out that magical little purple circle, also known as ovulation? Can't get anything past you!
So, what exactly is happening to cut out that follicular phase? Here are some basics you need to know:
There are actually 4+ hormones involved in your natural menstrual cycle: Indeed, there's estradiol (estrogen) and 3 other "players": progesterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH).
Different hormone "players" have different times to shine. These 4 hormones work in harmony, much like a basketball team (-1), running with the ball, passing it down the court, or blocking offensive players to make a basket.
None of this really goes down on a perfect 28-day cycle, just like no basketball game actually lasts 48 minutes.
Oh - and one last crazy ground rule in this game? Your period is actually the very first part of your cycle.
To illustrate how this looks across your cycle, here's a cutie little graph:
Without getting too complex, it's obvious to see that these four hormones work in conjunction to make stuff happen throughout your menstrual cycle, not just at the end when you bleed. In fact, if you're saying to yourself "that section to the left of that black middle ovulation line almost seems more dramatic" you'd be correct.
Remember, ovulation is the big event halfway-ish through your cycle when you release a mature egg (ova) that could be fertilized by sperm to make a baby. It makes sense that the lead-up to releasing an egg is quite action packed.
The Follicular Phase
The follicular phase starts when you're bleeding, shedding your old uterine lining to make way for the new. At this stage, all four of your menstrual hormones are at a pretty low level in your body. If we're (still?) using the basketball metaphor, this is a tournament and they're recovering from their last game, sweating it out with towels on the bench. However, one hormone FSH is that kind of player that just can't sit still. She's warming up on the sidelines pumping up the rest of the team to start the cycle anew.
After your period ends and you have that fresh new uterine lining, your hypothalamus (tiny little piece of your brain - we'll call her the referee) decides it's time to play ball again and tells your pituitary gland (coach, also in your brain) to start releasing some FSH. Follicle stimulating hormone's job is, quite literally, to stimulate the follicles to move towards becoming mature eggs that can be fertilized by sperm. After FSH has started its work on a few dozen ovarian pre-egg follicles, these baby eggies then stimulate the production of estradiol and LH - only about three days behind FSH.
FSH and LH are actually the most important hormones to ovulation, as they get your egg (ova) mature enough to be released from its follicle. FSH, LH, and estradiol all release to varying degrees in the follicular phase, but nevertheless spike in sync with one another - right before you ovulate (reminder image of how these levels look, graphically, below). Exciting times, eh?
When those hormones suddenly drop dramatically, your ova is ready to shoot out of its mature follicle through the ovarian wall into your abdomen, like a debutant making her way onto the dating scene a la Bridgerton.
This is, obviously, the event that scientists want to prevent from happening when they put you on HBC. If you don't release an egg, it can't be fertilized. Simple supply and demand stuff, right?
Hold that thought. We'll come back to it. For now, I want to end our discussion of the follicular phase with the cracked and broken egg shell. That follicle we talked about. What does your body do with it? Just discard it? Throw it away? Of course not! That would be too simple and clean cut.
Crazily enough, your body doesn't actually just "throw out" the shell, but rather transforms it (through the powers of LH) into a new important actor: the corpus luteum.
And, in fact, the corpus luteum is critical "go-between" crossing that black dividing line of ovulation and bridging between the follicular phase and the luteal phase.
The Luteal Phase
Progesterone really takes the spotlight during the luteal phase. A nesting hormone, progesterone's sole responsibility is to make your uterus cozy AF for the fertilized egg to implant and grow into a baby. It does this by sending signals to you that you're hungry and tired. You eat and sleep more and send lots of energy to your uterus.
And it's important to note that progesterone will work like this whether your egg is fertilized or not. Scientifically, progesterone production is really more of a byproduct of that egg "shell" (the corpus luteum) breaking down, than anything else. While progesterone levels climb and your uterus starts to get cozier, estradiol decides its time to rejoin the party for a second small bump towards the end of your cycle. Still, if there's no fertilized egg implanted in the lining of your uterus, it's game over for the corpus luteum. After a while, it completely disintegrates and your body runs out of progesterone supply, which signals the endometrial lining to shed (you bleed) and the cycle starts anew.
So...how does HBC work, then?
A long time ago, scientists figured out that with these hormonal patterns looking so different and really determining when you release the egg, they could actually manipulate the cycle to be in a sort of permanent luteal phase, by loading women up with synthetic progesterone and skipping ovulation entirely.
Remember, progesterone didn't really have a role to play during the follicular phase. And that's mostly because it's a byproduct of the corpus luteum - that egg shell. So, if your body takes in a synthetic high dose of progesterone (HBC) what happens?
Your body assumes that you've released an egg already and the corpus luteum is doing its thing. This means...and this is the punchline...you don't ovulate on birth control and you don't have a follicular phase at all. All you do on HBC is sit for 20 days in a luteal phase holding pattern and then bleed. No FSH, no LH, no egg maturation, and no release. No chance of getting pregnant.
But, is sending synthetic progesterone into your body and completely eliminating LH and FSH's roles a 100% good thing? More to come on that...(hint, it gets weird.)
See the continuation of this series: How Birth Control Works, Part II: Myths and Truths About Hormonal Contraceptives
*Resources:
For a brief and fascinating history of birth control and its important role in reducing maternal mortality, check out this resource from Planned Parenthood.
Major credit to my greatest teacher, Sarah Hill, PhD, who wrote This is Your Brain on Birth Control (a book I believe everyone should read, men included)
Also huge credit to Appendix C of Taking Charge of Your Fertility, by Toni Weschler Mph
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