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:

Method	Failure Rate
Nothing	85%
Condom	15%
The pill	8%
Copper IUD	0.8%
Tubal ligation	0.5%

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.