The Science of Birth Control Part 2

Last week we learned the biology behind the menstrual cycle.  This week, we’re going to learn how that biological understanding was used to prevent pregnancy ““ using some of the same hormones.

In order to brush up on how the menstrual cycle works, please see The Science of Birth Control Part 1: The Menstrual Cycle.

There are many methods of contraception; my main focus will be on hormonal methods.  However, for the sake of completion, I am also including barrier methods as well as a brief discussion of surgical sterility.

Barrier Methods

The contraceptive sponge


These methods include the diaphragm, spermicide, contraceptive sponge, cervical cap, cervical shield, female condom and male condom.  As indicated in the heading, these methods work by creating a barrier, blocking the sperm from entering the cervix.  Many of these methods, if used properly, can be very effective.  However, it should be noted that the male condom has a relatively high failure rate and natural versions (ex. lambskin) do not protect against sexually transmitted infections.



Hormonal Methods

The combined pill ““ This type of oral contraceptive pill contains both estrogen and progestin (progestin is a class of progesterone-like drugs).  Different types of combined pills use differing amounts of hormones and alternate members of the progestin family.

The mini pill ““ This pill only contains progestin and is ideal for women over the age of 35, women who cannot take estrogen, women who are nursing and women who are prone to blood clots. These progestin only pills contain different progestins depending upon the type of pill prescribed.

Vaginal Ring ““ A thin, flexible ring inserted into the vagina.  It contains the hormones estrogen and the progestin etonogestrel.

The NuvaRing

The Patch ““ A skin patch worn on the lower abdomen, buttocks or arm.  It contains the hormones estrogen and the progestin norelgestromin.

The OrthEvra patch

Depo-Provera ““ An intramuscular shot given once every 3 months.  This shot only contains the progestin DMPA and can only be given for up to two years as DMPA has been shown to cause reversible bone loss.

Depo Provera

The estrogen and the progestin suppress the release of follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland.  From the previous discussion on the role of FSH and LH in the menstrual cycle, you will remember that FSH and LF play an important role in the development of the follicle and the maturation of the oocyte.  Without the release of FSH and LF from the anterior pituitary gland, ovulation will not occur.  In addition, the constant level of estrogen and progestin cause a decrease in the amount of endometrial tissue being deposited around the uterus, meaning that a fertilized egg would be unable to implant.  Progestins also cause a thickening of the mucus at the cervix, creating a barrier that prevent sperm from entering the uterus.

Interestingly, the effect of progestins seem to be linked to the amount and activity of the specific progestin utilized!  While the progestin in the mini pill may not prevent ovulation, in the case of Depo-Provera, the progestin DMPA alone decreases FSH and LH levels, which then prevents ovulation, deposition of endometrial tissue and thickens the mucosal covering of the cervix.  Depo-Provera is effective nearly from the moment the shot is applied.

The combined pill, the vaginal ring, the patch and Depo-Provera prevent ovulation, decrease endometrial tissue deposition, prevent implantation and increase the thickness of cervical mucus.  The mini pill may prevent ovulation, decreases endometrial tissue deposition, prevents implantation and increases the thickness of cervical mucus.



Implantable Devices

Hormonal Intrauterine Device- The brand name of this device is Mirena. Mirena releases a low dose of the progestin levonorgestrel directly to the lining of the uterus for up to 5 years.  The exact method by which Mirena prevents pregnancy is not entirely understood.  However, it is believed that the low level of progestin causes changes in cervical mucus thickness and may alter ovulation as described in the hormonal section above.  The presence of the IUD in the uterus may cause chronic inflammation in the fallopian tubes and endometrium and likely fosters an environment that prevents fertilization and implantation.


Copper Intrauterine Device– The brand name of this device is ParaGard. ParaGard is an intrauterine device that releases small amounts of copper for up to 10 years.  The copper released by ParaGard causes chronic inflammation in the fallopian tubes and endometrium which creates a hostile environment for implantation of a bastocyst.  In addition, copper is directly toxic to the sperm and ova.


Implantable Rod ““ The brand name of this device is Implanon.  It is the size of a matchstick and is implanted subdermally in a woman’s upper arm.  Implanon releases a steady amount of the progestin etonogestrel for up to three years. Similar to the methods described in the hormonal section above, the progestin inhibits ovulation and thickens cervical mucus.



Surgical Sterilization

Surgical Implants – such as Essure or Adiana are permanent methods to ensure lack of fecundity.  In the case of Essure, stainless steel coils and PET fibers with an outer nickel covering are placed in the fallopian tubes.  These materials were chosen because of a history of scar tissue forming around those materials. The PET fibers cause a benign invasion of monocytes and other immune cells, resulting in complete occlusion or blockage of the fallopian tubes, preventing fertilization.  Sterilization is achieved in the case of Adiana by thermally damaging (burning) the lining of the fallopian tubes with radiofrequency waves followed by implantation of a silicone matrix into the damaged area of the fallopian tubes.   Occlusion occurs as fibroblasts fill the spaces between the silicone matrix.


Tubal ligation – A tubal ligation is considered major surgery.  The fallopian tubes will be altered at their thinnest point (the isthmic portion), closest to the uterus.  The fallopian tubes are most commonly cut and sutured in such a way that they cannot reattach.  The fallopian tubes can be banded or clipped which causes blood loss and scarring and eventually occlusion.  The fallopian tubes can also be cauterized.

Tubal ligation


Vasectomy – The vasa deferentia of the male is severed, which prevents sperm from entering the ejaculate.  Typically, the vasa deferentia are cut, cauterized or clipped.  Vasectomy is considered an outpatient procedure.


Emergency Contraception

There are other types of emergency contraception, including implantable contraception, but Plan B, as the most well known, is the method being focused on here.

Even though emergency contraception contains the progestin levonorgestrel, Plan B is believed to work through a mechanism of action a little different from other hormonal birth control methods discussed above.  This may have to do with the concentration and activity of levonogestrel itself. It has been shown that Plan B does not effect the implantation of a fertilized oocyte and does not interfere with the endometrial wall in anyway, contrary to popular belief.  Therefore, Plan B cannot, in any way, be called an abortifacient.  Rather, studies have shown that instead, Plan B impairs the ovulatory process and the function of the corpus luteum, likely though similar methods described above.  In addition, Plan B also seems to effect sperm migration and function.

Plan B



Progesterone released from the corpus luteum is necessary during the maintenance of a pregnancy, especially through the first 9 weeks.  One of the major roles of progesterone  is to maintain the uterine lining necessary to provide nutrients to a developing embryo.  Mifepristone (RU-486) is a competitive progesterone receptor antagonist.  What this means is that mifepristone more strongly binds the progesterone receptor than progesterone does and will actually compete and win (in a manner of speaking) for the binding site on the receptor.  The term antagonist means that mifepristone will bind to the progesterone receptor but will have no biological effect.  Once all the progesterone is competed off the receptors by mifepristone, menstrual bleeding will commence and the uterine lining will slough off, including any implanted material.  Additional treatment with prostaglandins aid in contraction and cervical softening.



Of course, there are methods of contraception I have not covered, mainly due to time and space issues.  Lack of coverage does not reflect upon the importance of those methods.  I hope this series on the biology of the menstrual cycle and contraceptives has been informative.  Additionally, I hope you have as much fun reading it as I had putting it together.


Again, a special thank you again goes out to Jen R. L. Disarray for suggesting the topic in the first place!


Did I get something wrong? Have a question? Send me an email at


38 replies on “The Science of Birth Control Part 2”

I think I should share that I’ve been having trouble finding a doctor that will put in an IUD for someone who hasn’t had children. Everyone cites the expulsion rate for women who haven’t had children, but yet I know several childless women who love their Mirena.Has anyone had this experience with their docs as well?

According to my OB friends (and MIL), the reasoning behind not putting in an IUD in a young, never-been-pregnant woman is due to liability. There have been some studies that show IUDs to cause problems with fertility later on. These studies were all done quite a while ago and it’s believed that the IUDs on the market now do not cause infertility.

However, it is possible that if an OB/GYN was to implant (I’m not sure if this is the correct term) an IUD in a young, never-been-pregnant woman and that woman went on to later have fertility problems, then that doctor could be sued. Even if it appears that the issue is unrelated to the IUD, it’s nearly impossible to rule out that the IUD entirely. That’s why Mirena is always advertised to women who have previously had a child, so the doc won’t be blamed for later fertility issues.

I read quite a lot about these methods of birth control and what I read about Mirena stated that about 80% of women are fertile within 2 months after removal.

The reason Mirena was originally marketed toward women who had previously given birth, at least according to my research, was that birth has the effect of actually changing the structure of the cervix – it opens it a little, making an IUD far easier to implant and greatly decreasing the risk of uterine puncture.

Many physicians now dilate the cervix and prescribe painkillers prior to implantation to ease the process.

Though, I guess it is possible that some physicians are worried about litigation as well.

This is going to sound dumb, but I had no clue how an IUD looked in respect to a uterus- I had never thought about it. So thanks for that info (seriously).

For some reason I pictured my uterus as much larger- like, car sized. I realize this is not the case.

You have a good point. I included RU-486 for several reasons. First, in my original article, I spoke about family planning. Even though RU-486 isn’t a form of contraception, it does have a role in family planning and I do think it should have been included.

It also falls under the umbrella of “Birth Control” which, unfortunately is fairly vague. I also thought the method of action that RU-486 worked was very interesting and contrasted with the action of most of the hormonal methods, including Plan B.

Thanks for pointing out the fact that technically, RU-486 is not a contraceptive, as they PREVENT pregnancy. I’m sorry that was not more clear.

I have a question about this:
The mini pill may prevent ovulation
So if it doesn’t I’d have regular periods every 28 days or so or would it more likely be spotting at random intervals?
How serious is the under 35 thing for it? I’m in my early 20s and my GP suggested it after I mentioned I used to have migraines with aura.

I had some really bad experiences with the combined pill, there’s no way I am trying out any of these again (plus the aura thing). So it’s between the mini pill or an IUD and while I don’t mind heavier periods as long as they are somewhat predicable I think I would get annoyed with random spotting.

I think the mini pill is more prone to random spotting. AFAIK there’s no placebo week so no regular withdrawal bleeding.

A friend of mine can’t take the combined pill because of her migraines, but her doctor was happy for her to use the ring, as it’s a much lower dose of estrogen, and more localised. Could be something to look into for you?

On any of the hormonal methods of birth control, there is no actual menstruation. What you have is withdrawal bleeding.

And the ovulation means that an egg is released, there are other methods described above – no endometrium, thickened cervical mucus, that prevent pregnancy. And the over 35 thing is a recommendation.

Women over 35 on combined pills have a higher incidence of stroke and heart disease. That’s what that recommendation is in regards to.

ETA: My sister has hemiplegic migraines and menstrual migraines – she’s on the ring and keeps it in during her ‘off week’ as she goes through hormone withdrawal (complete with migraines). It has a low enough dosage, as QoB suggested, that it doesn’t trigger migraines but enough that it doesn’t cause withdrawal. Mirena also has a very low dosage of hormones, if you are interested in going that route.

I didn’t have a lot of luck with hormonal methods, and I tried at least five different types of pill. The diaphragm was the easiest thing next to condoms, and I always wanted to be the one making DAMN SURE the BC worked. I had to experiment with spermicides to find one that didn’t irritate, but once I did I was so happy with my diaphragm. Plus, it looked like a hat, so I could get joke mileage out of it.
My only dislike: I accidentally left it in a hotel bathroom and I’ve been embarrassed as hell ever since.

I didn’t have any luck with the pill either, and I tried the patch (which was TERRIBLE)and the ring, which wasn’t bad. I’m happily a Mirena user.

Unfortunately, I have a mild-moderate allergy to latex. I’ll leave to your imagination how I found THAT out.

Is the diaphragm made of latex?

Mine was made of latex, and it was the most ungodly shade of pink you can imagine. I think they’re making them with other materials now? The good ole diaphragm doesn’t seem to be as popular these days, even when I used it 10 years ago I was the only woman in my group who used a barrier method.

I did not know Depo can only be given for two years. I know that’s a really common treatment for women with severe disabilities to prevent menstruation. I hope the women I know who took Depo have doctors who are aware of this side effect.

Does Mirena prevent menstruation too? Because one of the downsides of just plain IUD’s is that periods get a little bit more suckier all around. My periods are sucky by themselves so I don’t want to do anything that makes them worse. I’m thinking of going on a hormonal BC, just to control my cycle, as soon as I’m off my parent’s health care (they won’t let me go on the pill) and on my Grad school’s. I’ve heard many stories about side effects from the pill (weight gain, acne, mood shifts, loss of sex drive etc) and then there’s my mother’s unshaking belief that oral contraceptives cause cancer (which isn’t quite true, I know), so I’m a bit unsure about what to do.

Any advice, anybody?

On hormonal contraceptives, you don’t actually menstruate. What you do is called withdrawal bleeding where the lining of your uterus weakens enough to bleed a little. I have Mirena and I’ll spot for a couple days 1-3 times a year. I do get crampy very occasionally and I break out and PMS, but other than that, Mirena has been fantastic. Other than implantation. That was a nightmare.

As for the stories you’ve heard about bad side effects from the pill – it varies tremendously from person to person, and it is possible for many women to find a pill that works well for them. It took me one try – I spent 3 months on Seasonique and broke out constantly and terribly, but I switched to Lo-Ovral and I have no negative side effects. This isn’t meant to tell you to go on the pill, but know that the pill doesn’t inevitably mean lots of crappy side effects for everyone. :) (As for cancer, I believe the pill has been shown to help prevent certain types of cancer, like ovarian cancer.)

I’ve been off and on hormonal BC since I was 17 to control an irregular cycle, and I think the only bad part about the pill for me was the breakthrough bleeding in the first few months. I was living in a dorm at the time and would wake up with blood EVERYWHERE, which as a terrified, socially reclusive teen was basically the Worst Thing Ever. It subsided after a few periods, though, and we’ve been getting along happily ever since.

There’s been an adjustment period for my skin as well when I’ve gone off and on it, bad breakouts for a few weeks but nothing horrible. The breakthrough bleeding never happened again.

Mirena stops any bleeding for about a third of the women on it. The rest can get spotting or irregular bleeding.

In your position, I’d try a short term hormonal contraceptive first – pills, ring, patch. Given that you haven’t taken any hormones before you don’t know how they’ll affect you+better not to invest in something long-term and then have to have it out a few months later, or in the case of Depo just having to wait for the injection to wear off.

I’ve had great experience with the combined pill: have been on two types, very few side-effects (occasionally sore boobs), lighter periods, and so far foolproof contraception. If you do go for the pill, start out with a lower dose one and see how that goes for a few months.

Love this.

I’m going to write on my blog about my Essure experience next week. I did sign a release that if for some reason they couldn’t implant the little coils that, while still under, they would do a laproscopic tubal ligation. So, depending on how I end up getting sterilized, I can’t wait to tell you all about it!

I have a question about the practice of stacking hormonal birth control: What is the purpose of taking a break and allowing a period every 3 months? Is it a failsafe thing to check to see if all systems are functioning? Is it meant to be a chance to see if you are accidentally pregnant so you don’t end up on one of those “I never knew I was pregnant” shows at 9 months gestation?

You actually don’t have a period when you’re on the pill. What you have is withdrawal bleeding. This is when the lining of the uterus weakens just enough for bleeding to occur.

There is no medical reason to have a faux-period while you’re on the pill.

I use Mirena as my form of contraception. I don’t have any regular bleeding, but I do occasionally spot. I have heard physicans (WHICH I AM NOT) say that having a time set aside to bleed lessens occasional, random spotting but I have no idea if that’s actually true.

I do this! I think there are a few different reasons – going forever without a withdrawal bleed makes some women nervous and I think some women like to feel like they still have a schedule. I’d imagine another reason is because of breakthrough bleeding – it’s extremely common when you first start stacking, and taking that week off seems to let things reset. From my experience, and this could certainly vary and maybe Dorilys knows the sciencey reasons about why this would happen: the first year that I stacked my pills like this I’d have breakthrough bleeding 1-2 weeks before my placebo week, and once I’d start bleeding while on my active pills, back when I first started stacking, it would continue until I take a week off. Really, though, in my unexpert opinion, I think it’s just kind of arbitrary – the schedule the pill puts you on when you take it in the “traditional” way is arbitrary too; it’s supposed to mimic how your cycle would probably be if you weren’t on it, but there’s not really any reason that you have to take a break on week 4. (As long as you’re not taking one any sooner.)

Okay. So according to my quick search of Google and the literature, there can be multiple causes of spotting or breakthrough bleeding.

a)Taking the pill at different times of the day.

b)The dosage of hormones is not high enough for you. Estrogen causes the endometrium to grow and progestin stabilizes it and if there isn’t enough for your body specifically, then you may get spotting.

c)The type of progestin in your pill.

d)Cycle length – it may be that the longer the cycle, the more difficult it is to maintain endometrial stability and the more likely there is to be spotting. Therefore stacking or the 3 month pill will result in more breakthrough bleeding.

e)Other factors such as smoking or several types of cancer.

This is mostly from the interwebs, since it was a quick search. DON’T HATE ME IF I’M WRONG.

I COULD NEVER HATE YOU. I know that most women who stack have breakthrough bleeding for at least the first few cycles – what I’m curious about is what you mentioned above, whether taking a week off does indeed help lessen your chances of having breakthrough bleeding and how why I would stop bleeding after taking a placebo week but not before. Maybe I’ll put that on my list of questions to ask at my next gyno appointment. :)

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