The Science of Birth Control Part 1: The Menstrual Cycle

Birth control comes in many forms: barrier, hormonal, implantable, and permanent.  Many women (and men) utilize birth control as part of their family planning strategy, but how many of us understand the science behind the medication we’re taking?  I’m here to explain it for you.

First, a diagram of the female reproductive system, to orient us:

The female reproductive system


In order to properly understand how the various forms of birth control work, we need to understand the biology behind the menstrual cycle.


On the first day of your menses, in your uterus, you slough all the endometrial tissue built up during your last menstrual cycle.  The really interesting stuff all begins in your brain.  Your anterior pituitary gland begins secreting a couple of gonadotropic hormones.  Let’s break down the vocabulary.  “Gonad” – just like you’d figure as a kid – referring to ovaries or testicles.  In this case, obviously ovaries.  “Tropic” usually means having an affinity for or being directed to; so scientists made a long word saying that the hormones are targeted toward your sex organs.

Panel A: The release of gonadotropic hormones in your brain. Panel B: The maturation of an oocyte and the development of a follicle. Panel C: Hormones released by the follicle/corpus luteum. Panel D: What's happening in your uterus.

First to increase is a hormone called follicle-stimulating hormone (FSH).  Any follicles in your ovaries which are developed enough to respond to the hormone begin growing both in size and number of cells.  You can see this in panel A where the increase in FSH is shown in the orange line and in panel B where egg maturation and follicle development is chronicled.  As the increase in size begins, your anterior pituitary begins releasing a hormone called luteinizing hormone (LH), shown in panel A as the red line.  The primary purpose of LH is to break down a meiotic block (which means that the oocyte was previous not able to divide – LH was all, “You shall divide!”). What this means is that the oocyte can asexually reproduce, or as it is more commonly stated, the cell divides*.  The oocyte is then halted in meiosis II until fertilization. At this point, the oocyte is ready to be ovulated.

FSH and LH are now stimulating the follicle to produce estrogen (the blue line in panel C) , which is the major player at this stage.  Estrogen is causing your uterus to produce nutrient-rich tissue full of blood vessels and glands, in preparation for the implantation for a blastocyst (you can see the rise in estrogen production and the deposition of tissue in panels C and D).  It is also thinning the mucous of the cervix to allow sperm into the uterus.  Finally, a very delicate interaction between estrogen, LH, FSH and gonadotropin releasing hormone (GRH) promote the actual ovulation of the oocyte.

LH, which to me seems like the star of the menstrual cycle, has one more role to play.  It converts the now-empty follicle to the corpus luteum.  The only take-home message about the corpus luteum that we need to know is that it is a hormone-making factory. The corpus luteum levels off estrogen production and now begins to produce progesterone (the purple line as seen in panel C).  Progesterone inhibits FSH, making certain no other follicles mature.  It also further prepares the endometrium for blastocyst implantation.

Blastocyst fertilization and implantation.

If no blastocyst implants, your cycle begins anew with the shedding of the endometrium.



A note on menstrual synchrony:  Missbananafish sent me an email asking about whether or not menstrual synchrony really existed and could possibly be shared in an internet community such as Tumblr.  The fact that the length of the menstrual cycle varies between individuals seems to argue against the existence of menstrual synchrony.  However, there are two major theories regarding synchrony.  First is the McClintock theory in which the conclusion was that pheromones cause women’s cycles to synchronize.  However, there are many problems with the way the study was collected and the data was analyzed.  Depending on the method of analysis, different conclusions can be drawn.  There is another theory that synchrony seems to happen in groups of women who share similar stressors.  It is well known that stress can lengthen or shorten a menstrual cycle and therefore make it seem as though women’s cycles are synchronizing.




Oocyte maturation

*What is this? The oocyte asexually reproduces?  Yes. This is a bizarre part of the trip the oocyte makes toward maturity. Meiosis I, or the first cell division, the first step of oocyte maturity, occurs before birth of a woman, the second step occurs right before ovulation and the third step occurs after fertilization.

All this dividing is absolutely necessary to make a haploid (meaning it only has half the normal number of chromosomes) cell.  That way, an oocyte can meet up with a sperm, which is also haploid, and make a new cell that is has the normal diploid complement of chromosomes.  This way we don’t end up with mutants or non-viable offspring.  The polar bodies are the cells in which the unused portions of the chromosomes are popped into (the scientist term for this is segregated)  the zona pellucida and do not serve a purpose for this discussion.





Now that we understand all the menstrual cycle players, next time, we’ll learn how scientists figured out how to inhibit them to prevent conception.


Special thanks go out to Jen R. L. Disarray, who sent me an email, asking if I’d explain the biology of birth control!


Did I get something wrong? Have a question about science? Email me at

11 replies on “The Science of Birth Control Part 1: The Menstrual Cycle”

This is great! One thing I would add is that the charts are the “average” — not everyone has a 28 day cycle, so not everyone ovulates right at day 14. So don’t just use barrier birth control when a generic chart says you ought to, if you are trying to prevent pregnancy. One of my good friends got pregnant twice in two years because of this fact, despite the fact that she and her husband both have Ph.D.s in biology– they just forgot that variation applies to humans, too. But they have two very cute “surprises”.

Very cool! One more point on menstrual synchrony on the internet – it’s highly subjective to observational bias. For mathematical simplicity, let’s say you have 120 menstruating internet friends with an average cycle length of 30 days (about a month). Whatever day you start your period, it’s statistically likely that 3 of your friends started that day as well, with another 16 friends starting within 2 days of you. You notice this because it’s relevant to you, but you don’t notice the other 100 friends who aren’t starting their period. Ergo, cluster! Obviously there may be pheromones or environmental factors that affect people living in close quarters, but online, it’s probably all in your head. Though it’s still comforting when we’re all hormonal together!

In case anyone was interested, I asked a visiting scientist why oocytes go through two separate meiotic divisions.

He said there was no data to support his supposition and he hadn’t read any studies regarding my question. However, he believed that given the long period between meiosis I (during the development of a female human in the womb of it’s mother) and the long period before the oocyte might be fertilized, meiosis II might be a type of check point to make certain that the oocyte is still good – not mutated by radiation or drugs or anything else. If the oocyte was not functional, it would not progress through meiosis II and would not be able to be fertilized.

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