Women and STEM: a guide to the menstrual cycle

Note: To understand what our hormones do and how birth control uses hormones to prevent pregnancy, please pair this article with “Anna’s In-Progress Guide to Hormonal Birth Control.”

That’s right, it’s time to finally tackle this beast: How in the world does the period cycle work? The truth is, even though very few menstruators actually know what’s going on in their brain and body on any given day of the month, it’s really not so complicated. And understanding the hormonal mechanism behind the scenes really helps explain some of the less pleasant symptoms that can happen along the way.

For much of my discussion, I’ll be speaking in averages: Everyone’s cycle and symptoms are unique, but understanding the basic structure can help you apply the significant concepts to your personal cycle, no matter how beautifully (or annoyingly) irregular it may be.

The average cycle lasts 28 days, or one month. The first five to 10 days are the actual “period,” or the bleeding, and ovulation happens around day 14. I’m going to start our menstrual story on that day.

Ovulation is the release of an egg from the ovaries into the uterus. It is the day that egg-havers are most likely to get pregnant, because the egg is so new and readily available. It is also the time in which many menstrual hormones are at their highest levels, including estrogen, testosterone, luteinizing hormone, and follicle-stimulating hormone.

What does that mean for your body? We’re thriving. First and foremost, the body loves estrogen. It clears your skin, strengthens your bones, and likely puts you in a great mood when combined with the other hormones. The increase in testosterone, which might be confusing because most people associate testosterone with people who don’t have menstrual cycles, works as a balance against the increased estrogen. We always have a low level of testosterone in our bodies, but it would be an awkward outlier if it remained low during ovulation.

Luteinizing hormone (pronounced loo-ten-eye-zing) is a regulator. More specifically, it promotes the production of progesterone, which is a key player in pregnancy. Increasing progesterone makes your body more ready to carry another human being, and as you can imagine, that’s a very healthy state.

Follicle-stimulating hormone (FSH) plays a slightly more technical role. FSH incites the ovaries to do the actual releasing of the egg, which lives in a follicle. When the follicle is stimulated (hence the hormone name), it prepares the egg to exit the ovary through the fallopian tube and into the uterus.

So now we have an egg in our uterus and we’re all ready to fertilize it and make it into a baby. Great! Except, chances are you’re not getting pregnant every month, so what happens to that egg? It stays there for a while, and as your body starts to notice that you’re not using it for anything, it gets ready to get rid of it before it gets too old to use. The lining of the uterus — or the endometrium — builds up a new layer of blood vessel cells and other types of cells. Spoiler alert: That layer isn’t going to stick around for very long.

Hormonally, though, the body hasn’t given up on the concept of pregnancy completely. Ovulation is a hormonal spike, so all the hormones I talked about earlier decrease rapidly after the egg is released. This is why some people experience spotting, or light bleeding, in the middle of their cycle — the decrease in estrogen is to blame. All of the hormones I talked about in the context of ovulation are now at low levels except for estrogen, which begins to increase again in the last few weeks of the cycle (or the luteal phase). And I already told you that progesterone has been triggered by LH, so there’s an increase in that, too. Things are going pretty well for the body for a little while, until the month is over and everything drops all at the same time.

Welcome to PMS, or premenstrual syndrome, the name given to a horrifying symptom soup that happens before the period and is caused by the complete hormone drop. Low estrogen causes (amongst a long list of other things) sudden mood changes, acne, and increased salt retention that leads to bloating. Low progesterone signals that time is up for our egg, which is by now sitting in a thick layer of endometrium. Getting rid of that layer requires muscle contractions that are to blame for period cramps. The layer itself is shed as blood during the menstrual period. Hormones remain at a low level, with estrogen gradually increasing until ovulation.

This might seem like a mess of changes, and it is certainly a complex system. But it’s also highly evolved, meaning that, like it or not, everything you go through pushes us all towards the goal of successful procreation.

That said, I’ll end with some food for thought: If we’re including estrogen (which does not prevent pregnancy) in our birth control treatments, why are we giving women steady doses all month long? Could we not take a hint from nature and grade the dosages to replicate the cycle without ovulation, and would that not result in healthier birth control outcomes?