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.

I kind of see what he means.

In defense of NFP, however, you're comparing that method to other methods, which use the same comparison, i.e. effectiveness over one year. People think that if they use The Pill they won't get pregnant. They really believe that illusion. There is no such illusion with NFP. When you use the Pill and you get pregnant, you feel betrayed. When you use NFP and you get pregnant, that's just part of the package.

The other thing about NFP is that it's really as effective as you make it. It's theoretically possible not to have ANY sex after ovulation (to be perfectly safe). Now that probably wouldn't make the man happy, but if you did it that way, your chances of getting pregnant are virtually zero. I suspect that's probably not something you'd want to do over the course of 25 years. On the other hand, if you have a spate when you absolutely cannot become pregnant, it's perfectly possible to plan for that, too. In fact, I would trust NFP over anything else if that were my case (not that I'd use anything else.) If you have a regular cycle, and you know what days are safe and what days aren't, you can make pregnancy virtually impossible, which is different from other methods of contraception because you don't know ANYTHING of what's going on within you.

The thing about NFP is that it was never meant to be a perfect method. NFP is just as much a philosophy as a method. The point is that babies are not "failures" they're "surprises".

There is a subculture of NFP enthusiasists among Catholics. They think it's the greatest thing since sliced bread. I'm happy for them. I'm not one of them. I think the claims made about NFP is a bit over-stated. It works.It's moral. That's all that matters. But it's not a miracle. It has benefits. But not all couples appreciate them. I know that the "honeymoon effect" sounds nice to women, it's not so nice to men.