Friday, April 22, 2011

Birth Control Math

I learned about contraception at a reasonably early age (around 12, I think); I know that many kids are not so lucky. I don't recall where I got my information, but it was probably a combination of health class, my parents, and these pamphlets I got in the mail along with feminine hygiene product samples. In any case, I knew about condoms and the pill and that they could protect against pregnancy.

On the other hand, there was relatively little discussion of the odds. Numbers are important; to make informed reproductive decisions, you need to know how likely you are to get pregnant from a single sexual encounter, how likely you are to get an STD, and how effective the various forms of prevention are.

I still don't have the answers to all of these questions, though I have been able to find some other usable data (odds of getting pregnant in one year of unprotected sexual activity? 85%. Scary.) When I first saw exactly how effective the various birth control measures were, it took my breath away. If you use condoms, for instance, you've still got a 15% chance of winding up pregnant by the end of the year. This means that "use a condom" is woefully inadequate advice.

It gets worse when you consider the cumulative probability of getting pregnant over multiple years. Let's stick with condoms for the moment. The probability that you're going to avoid getting pregnant that first year is 85% (or 0.85). But the probability that you're going to avoid getting pregnant two years is 0.85 x 0.85. Each year, multiply by 0.85 again. That number gets small fast. After 5 years, it's more likely than not that you will end up pregnant at least once. The longer you play the odds, the worse it gets. In fact, if a person with a uterus of average fecundity uses only condoms for the 30 or so years they are ovulating, they can pretty much count on getting pregnant at least once:

Of course, we all know that condoms are not the only way to prevent pregnancy. Let's next compare a few different methods, the effectiveness of which I have simply copied from Wikipedia:

MethodFailure Rate
The pill8%
Copper IUD0.8%
Tubal ligation0.5%

The failure rates listed above are "typical use failure rates", or the percentage of the time that people using each method get pregnant after a year. With so-called "perfect use", the effectiveness of many of these methods improve, but are you really going to choose your birth control assuming that you're going to be better at it than everyone else? I'm sure as heck not.

Anyway, for each method, you can repeat the analysis I did above:

Or, if you prefer equations, for a birth control method with a failure rate of x% used for y years, the probability that you'll get pregnant at least once during those years is given by P%, where:

If you're just trying to delay pregnancy or not get pregnant quite so often, then great, these methods should work for you, but if you are, like me, dedicated to childlessness, these odds are not good enough. So, the next obvious thing to try is doubling up on methods:

I realize that most of my readers are adults and thus probably have made their birth control decisions. But if you're in a position to educate youngsters, please share this information with them. The more you know, etc.

There's a second point I want you to take away from these plots. Many of the most effective methods, such as IUDs and sterilization, are regularly withheld from young and/or childless women by doctors who won't believe their patients really don't want children. This is a symptom of our reluctance to let women make their own reproductive decisions, and it means that the most effective methods (and the methods that are most difficult for a partner to tamper with) are the hardest to obtain. The pill and the condom, both temporary methods that must be continually reapplied, are thus the most common choices in the U.S.

Many a cautious individual will choose to use both the pill and condoms. Even if these methods are continually used from menarche to menopause, the purple curve in the above plot shows that there is a 30% chance at least one pregnancy will occur during that period. At that point, to remain nulliparous, they will need to obtain an abortion.

To continue this thread of logic, this means that even if everyone doubles up on birth control methods, there will still be a not-insignificant need for abortion. And that assumes everyone can double-up; in reality, not everyone can take the pill or use latex condoms. Even if everyone in the country is extremely responsible about using birth control, there will still be a substantial need for abortion. I do not want to hear about how abortion should be rare. Unless your ideal involves widespread, compulsory sterilization, I just don't see how that's possible. Abortion is an important health service for everyone with a uterus, and should not be "rare" unless significant advances are made in birth control technology.


  1. Great piece, Kyrie. I was going to post a useful comment, but you anticipated all the questions I was going to ask. So, you'll have to settle for mere praise. :-)

  2. Heh. I think I can handle that :)

  3. Excellent work, thanks for putting these numbers out there in black and white!

  4. My first post!

    *Many of the most effective methods, such as IUDs and sterilization, are regularly withheld from young and/or childless women by doctors who won't believe their patients really don't want children.*

    Ryan and I have pretty much decided no on the whole having children debate (we kind of decided this 10 years ago, and 10 years later we think we're certain). We want Ryan to have a vasectomy, but have not yet approached a doctor. We've been putting it off in no small part because I'm sure we're going to have to deal with convincing a doctor that we don't want kids, EVER. I'm not sure if it will be harder or easier to convince a doctor a man wishes to be sterilized over a woman. Maybe we should do a side by side comparison and let you know.

  5. Oh, I have a feeling it will be harder for you. It is generally accepted that men have bodily autonomy and the emotional intelligence required to make decisions for themselves, but LOTS of people don't feel the same way about women. And on a more practical level, a vasectomy is a much less major operation than a tubal ligation. If you run the experiment, do let us know!

  6. @Damsel: I hope you don't experience this, but doctors can be pretty fucked up towards male sterilization, too. Thomas Macaulay Millar wrote a pretty fantastic post about his experience ( I think it's always worth reminding folk that a person's medical decisions are their own. Many couples like to share the decision-making, but they are totes not obligated to do so.

  7. Er, that's not meant to be a criticism of you, btw. Got fired up about body autonomy issues while rereading Millar's post.