The Effectiveness of Natural Family Planning (subtitled: 99% ... really?!?)

I wish people would stop quoting effectiveness rates for Natural Family Planning.  It seems to be the first “selling point” offered to apprehensive contracepting couples.  I have two reasons for not jumping the gun on these numbers,

First, the numbers vary quite a bit from study to study.  Part of the variation can be explained by the difference between “method effectiveness” and “user effectiveness.”  Here is how the effectiveness studies are done.  They take a group of women (to keep our numbers simple here, let’s assume it is 100 women) who do not intend to get pregnant, and they have them use a particular method of natural family planning for one year.  At the end of the year, if a women experiences an unintended pregnancy, this goes down as a “failure.”  (Yes, I know, a baby is never a failure, but a success; lets put aside the polemics for one minute - I’m on your side, remember.)  If a couple decides to intentionally get pregnant during the year, they cease to be a part of the data set.

“Method effectiveness” only takes into account couples who claim to be using the “rules” perfectly.  “User effectiveness” takes into account all couples, including those who admit to bending the rules, engaging in pregnancy achieving behavior on days that they suspect might be during the fertile phase (even though they still do not intend to become pregnant).  As you can imagine, the “user effectiveness” rates are lower than the “method effectiveness rates.”  Unlike contraceptive methods, following the “rules” of natural family planning involves self denial.  Thus, couples are more likely to bend the rules in natural family planning than they would if using contraception (where being the rules would amount to forgetting to take the pill on a given day).  Moreover, even method effectiveness can vary from study to study, in part because couples are treated as “perfect users” based only on a survey that asks them if they knowingly violated or may have violated the rules.  Obviously, survey data is subject to a tremendous amount of inaccuracy.  Are the couples aware that they bent the rules?  Are the couple embarrassed to admit that they bent the rules?  How “sure” or “unsure” does one have to be in order for it to count as a “rule break”?  All of these things lead to a range of method effectiveness ratings anywhere from 95%-99%.  (I have no doubt that studies exist even outside of this range; while I have read several of them, I certainly have not exhausted the literature on this topic.)

However, when people in the Church advocate for natural family planning by giving effectiveness ratings, they not only fail to mention anything about lower user effectiveness ratings, but they also tend to quote the highest method effectiveness ratings they can find.  I was even in a presentation once that boasted 99.9 percent effectiveness.  The most common rate quoted, probably because of its rhetorical value, seems to be 99%.

Even if people were to explain that (1) some studies found method effectiveness ratings that are below 99% and (2) user effectiveness, which may be a more reliable measure, is often considerably lower, I still have a problem with quoting the number in the first place.  To give the benefit of the doubt, let’s assume the most commonly quoted number: “99% effective.”  (That number, by the way, is what is known as a “Pearl index.”)  Remember how these studies were done.  They took 100 women, had them use the method for a single year, and out of the 100 women, there was a single unintended pregnancy (with a woman who was using the method perfectly).  Having the study last only a year is the single most important and overlooked fact in quoting the 99%.

What this means is, in any given year, assuming you are using the method perfectly to postpone pregnancy, a couple has a 99% of not getting pregnant.  The problem comes when we compound this effect over 2, 3, 4, 5, and so on, years.  Using basic laws of probability (multiplication of probabilities for independent events), a couple who is using the method perfectly for 2 years and trying to not get pregnant, has a (0.99 * 0.99) = 98.1% chance of success.  In other words, out of 100 couples who use the method for two years, 2 (not 1) will experience unexpected pregnancies.  So, is the method 99% effective or 98% effective?  Well, that depends on what you mean by “effective,” which is exactly my point.  As many know, this process of exponentiation works quickly, so that in five years, the method effectiveness drops to 95%.  In other words, if 100 couples use this method perfectly for five years, 5 out of the 100 will experience an unintended pregnancy.  Extrapolating to ten years, the method effectiveness is 90% (10 will experience at least one unintended pregnancy).  For the sake of a complete picture, let’s extrapolate this to a woman’s entire fertile life.  Suppose that she gets married at 25.  The average age of menopause is around 50, so the couple has 25 years of fertility together.  Let’s assume five kids (I’m Catholic, you know), and each kid produces a year of infertility.  (I know, there are a lot of other factors, but we are talking averages here just for the sake of a general picture.)  This gives the couple 20 years of fertility.  The method effectiveness rate over 20 years is 81.7%.  This means that out of every 100 couples using this method perfectly for their entire married life, 18 will experience at least on unintended pregnancy.  Now, this assumes the 99% one-year method effectiveness rate.  If we are talking user effectiveness rate, which the more generous measurements peg at 98%, the twenty-year effectiveness rate drops to 66.7%.  This means that 1 out of every 3 couples using natural family planning will experience an unintended pregnancy at some point during their marriage.  And you wonder why I think saying “NFP is 99% effective” is not completely honest.

Here is the good news.  Every study on the effectiveness rating for methods of artificial contraception that I have seen uses the same “Pearl index,” meaning the “99% effective” blurb has the same problems as the figures quoted for natural family planning.  Thus, while it is wise to avoid the actual number, “99%”, it is perfectly honest to say that natural family planning done perfectly is just as effective as the best methods of contraception done perfectly.

However, my second objection has nothing to do with the actual figure.  Quite simply, we do not use natural family planning because it is effective.  To advertise it this way makes it very difficult to answer the question, “Why isn’t this just another form of contraception?”  The reason we practice natural family planning is because it is morally permissible, and artificial contraception is not.  Don’t get me wrong, there are lots of arguments from both theology and philosophical anthropology that can go a long way in explaining why natural family planning fits in to God’s plan for the married life and contraception is contrary to this plan.  The arguments themselves are outside the scope of this post.  The point here is that the vast collection of arguments that support the Church’s teaching does not include effectiveness.  The method could be no better than “guess and check”, and the Church would still prohibit the use of artificial contraception, and we would still be bound to this teaching in good conscience.

99% effective?  Misleading and irrelevant.  My advice: stick with Scripture and theology and not with numbers.