Field of Science

Girlybits 101, now with fewer scary parts!

I was deeply considering a blog hiatus, dear readers, but sometimes you get hit with sledgehammers, and the only thing you can do to make sense of it all is to blog about it.

To make a long story short, Cat Marnell, the "Health" and Beauty expert at XO Jane (a website created by Jane Pratt, of Jane and Sassy fame), wrote a blog post or column or whatever about how New York is out of Plan B and this is very distressing to her because she doesn't believe in birth control and condoms or... something1. It is full of inaccuracies and misconceptions about hormonal birth control, and I am truly disgusted that a woman claiming to be a HEALTH (and beauty) columnist is spreading such filth without checking any of her facts. But I digress. I'm not here to lay the smack down because that's already been done (and better than I could do) by Skepchick and Scicurious (edit: and Kate).

No, what has my feathers ruffled is the part where poor Cat got so flustered by her own cycle that she dissolved into a fit of capslocked hysteria.

It makes me sad that (1) a HEALTH (and beauty) columnist refuses to discuss the actual health involved in the topic she's writing about and (2) this "OH GOD OUR BODIES ARE WEIRD" attitude isn't at all uncommon among girls of all ages. It sorta makes me wonder how much of her own basic physiology Cat actually understands. I think I can help out with that.

This post is a quick and simple Girlybits 101, using very small words and no gross or scary diagrams so that the eternally squeamish and uneducated like Cat Marnell can know a little bit more about their bodies without sending themselves into a hormonal frenzy. I know science is hard, dear, but that doesn't mean you shouldn't take the time to learn it, especially if part of your job description is to talk to women about their HEALTH (and beauty). However, I have to add a caveat. It is very likely that what I'm about to describe doesn't exactly fit your experience! The way women experience their cycles is very heavily influenced by their diet, their activity level, their relative amount of stress, their environment, their age, their reproductive status, how much muscle they have, how much fat they have, what types of activities they perform, where they live, etc. etc. etc. In short, a major theme of female reproductive physiology is that nothing is ever the same, not between women and not even within the same woman over time. My blogpal Kate even writes a whole blog on this topic, which I recommend the more intrepid and less-squeamish of you check out immediately for more information. Kate is a much greater expert on all things ladybits than I am, but I know at least enough to give you this quick non-scary overview.

And now, I present to you this Totally Easy And Not At All Gross Explanation of what happens with your girlybits each month, and how birth control and Plan B tie into the equation.

(1)    You have ovaries, which are a bit like testicles but on the inside. You probably know that ovaries make eggs, but they also make and release endocrines (more commonly known as hormones), which you can think of as messengers. Like letters in the mail. Or text messages, or whatever.

(2)    Every month, one of your eggs begins to mature. Your egg has a comfy little home inside the ovary that provides it all the things a growing egg needs. However, the ovary knows that one day the egg will have to leave the ovary and venture out into the fallopian tubes, where it might possibly meet a charming young sperm and settle down. The ovary wants to make sure that life is easy for the egg when it leaves, so the ovary makes estrogen (an endocrine).

(3)    Estrogen travels through the blood and goes all sorts of places doing all sorts of things. One of these jobs is to build a future home for the egg in your uterus.

(4)    After 14 days or so, the ovary has done all it can do to nurture the egg. Now the egg has to leave the nest and strike out solo. However, this story has a twist. If the egg doesn't meet a handsome sperm and fall in love within about three days, it will DIE!!! (No pressure.)

(5)    After the egg has left the ovary to go look for Mr. Right, the ovary continues to make estrogen and also progesterone, which is another endocrine that has a similar job to assist in building the egg's dream home.

(6)    At this point, one of two things can happen. The egg can meet Mr. Dreamy Sperm at the Fallopian Club, fuse with him, and move into her Uterine Dream Home, or she will die forever alone.

(7)    If the egg dies, the ovary eventually stops releasing estrogen and progesterone (it takes a few days because, uhh, the ovary doesn't have e-mail so it has to wait to get the news of the death through the mail), and a wrecking crew comes by and demolishes the dream home. If the egg fused with Mr. Spermy and becomes a zygote, she starts secreting her own endocrines to communicate with the ovary, telling the ovary what a nice sperm she met and that they're very happy in their new home. The ovary continues to release estrogen and progesterone so that they have a pleasant stay for 9 months, after which a baby magically appears.

Most forms of hormonal birth control interrupt this cycle so that step 4 never happens. The egg never leaves the ovary, so it never has to choose between marriage or death. Plan B can do the same thing if taken before the egg moves out. If the egg has already moved out, Plan B stops the egg and sperm (collectively called a zygote after they fuse) from moving into the dream home. Unfortunately, Plan B can't do anything if the egg and sperm have already moved in.

There, that wasn't so bad, was it? Hopefully some of you are even feeling brave enough to read the more technical version, which I promise is still very easy to understand (however it may include "scary" words like UTERINE LINING and IMPLANTATION and CORPUS LUTEUM, so, you know, beware or whatever).

(1)    You have ovaries. Ovaries are your gonads, or the sex organs that produce gametes, also known as eggs and sperm. Since you're a girl, you're makin' eggs.

(2)    The start of your menstrual cycle is the first day of your period. During and after your period, one of your eggs begins to mature inside one of your ovaries. (Contrary to what some people think, ovaries are not like testes in that they are constantly making eggs. Your ovaries are more like fancy storage compartments than anything else. You already have all the eggs you will ever have when you are born. The ovaries store these immature eggs until puberty, after which one egg matures and is released each monthly cycle.) The egg itself is inside a sphere of special protective cells called follicular cells, referred to collectively as the follicle.

(3)    While the egg is maturing, the follicular cells release estrogen into the bloodstream, which increases vascularization (i.e., it makes more blood vessels) in the uterine wall. More blood vessels means more nutrients for the uterine lining, which may be needed later if you become pregnant.

(4)    After 14 days or so, the egg is now mature and is released through the wall of the follicle and ovary into the fallopian tubes. This is called ovulation and happens because of a drastic spike in a different endocrine made by the brain called lutenizing hormone. The egg will stay alive for about 3 days, during which time it slowly migrates towards the uterus.

(5)    The follicle from which the egg was released begins to die. It is now called the corpus luteum, and it releases progesterone in addition to estrogen. Progesterone continues to build up the soft tissue of the uterine lining.

(6)    If sperm is present in the reproductive tract and fuses with the egg, it becomes a zygote and implants into the wall of the uterus. If sperm is not present, the egg dies.

(7)    If the zygote implants, it releases another endocrine called human chorionic gonadotropin, which communicates with the corpus luteum back in the ovary and indicates a successful implantation. The corpus luteum stays alive for the duration of the pregnancy and continues to produce helpful endocrines. If the egg dies, the corpus luteum also eventually dies and stops releasing estrogen and progesterone. Eventually, your body realizes that estrogen and progesterone levels have dropped off, and this triggers the start of your period. The uterine lining is shed, and a new egg begins to mature. Go back to step (1) and repeat.

Most forms of birth control contain synthetic estrogen and progesterone or just progesterone. By supplementing your body with these endocrines, you (ideally) stop ovulation from occurring. Plan B has a very very large dose of synthetic progesterone and can do one of several things. It can delay or stop ovulation all together, similar to regular oral birth control, but it can also stop a zygote from implanting by irritating the uterine lining (though this mechanism is under debate; see comment #2 over at SciAm). If the zygote has already implanted into the uterine wall, it has no effect.

I used to teach several hundred college sophomores a slightly more technical version of this every quarter.2 If they can understand it, by golly, so can YOU, Cat Marnell! If you have any questions after reading all of that, feel free to give me a call. I would never turn down the opportunity to teach someone about their bodies, as long as they're willing to stop holding their hands over their ears and yelling "LALALALALAGIRLYPARTS EWWW GROSSS P.S. SCIENCE IS TOTALLY HARD, YO" long enough to actually learn something.

1 I am not judging, by the way. I don't like condoms or hormonal birth control either. However I do feel the need to qualify that I am not on hormonal birth control because I have health problems that preclude me from being able to take it, not because I think they'll make me fat. (They won't.)

2 Without fail, every quarter I would have a male student come up to me after a review session or during office hours and tell me in private that he was very glad he took our class because he had so many questions about female cycles that he was just too darn afraid or embarrassed to ask anyone about. It was adorable and definitely made me feel like I had done something good for the world.

Originally posted at Scientific American on October 15, 2011.

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