Reader Question: Is NFP Really Effective in Avoiding Pregnancy?

In the comments on "Natural Family Planning: A Personal Perspective" a reader calls into question the 2007 German study that indicated that, if a woman has relatively normal cycles and the couple is practicing NFP correctly, the chance of a "surprise" is about 0.4 percent.

The reader points out that the established pregnancy rate for those practicing unprotected sex is 85 percent and argues that, therefore, the study cannot be right. In later comments, he suggests that this number is evidence that Catholic couples who use NFP have very infrequent sexual relations—perhaps as few times in their married lives as twice the number of children that the couple has.

Setting aside the reader's deliberate attempt at provocation (he uses the supposed infrequency of Catholic marital relations to illustrate that the Catholic Church supposedly "demoniz[es] sexuality"), is he right about the 85 percent pregnancy rate? And if so, does this undermine claims from both scientific studies of Natural Family Planning and NFP advocates that NFP is one of the most effective means of avoiding pregnancy?

A little knowledge is, as they say, a dangerous thing, and the reader is in firm possession of a little knowledge—namely, that the pregnancy rate among those who practice unprotected sex is 85 percent. That's true—but it doesn't mean what the reader thinks it means.

A woman who has relatively normal cycles is most fertile for essentially three days each month—the two days before ovulation and the day of ovulation. The day after ovulation, the chance of conception is 0 percent. Three days before ovulation, the chance of conception is 8 percent.

But even if a woman has unprotected sex on her most fertile days, her average odds of getting pregnant in any given month are 20 percent. Over the course of a year (which is how the pregnancy rate is measured), that translates into an 85 percent rate among women who have had unprotected sex during their most fertile days each month of the year.

The reader has made a very common mistake. He's taken the pregnancy rate and assumed that it means that every time a woman has unprotected sex, she has an 85 percent chance of getting pregnant. But, as we've seen, that's not true. At best, on average, she has a 20 percent chance of getting pregnant—provided she has sex on the two days before ovulation and/or on the day of ovulation. Outside of those days, her chance of getting pregnant drops dramatically.

So what relevance does that 85 percent pregnancy rate have to this discussion—both about NFP, broadly speaking, and about the German study?

If a woman has relatively regular cycles and practices NFP correctly, then she can easily avoid having sex on the two days before ovulation and on the day of ovulation. Indeed, every NFP method recommends avoiding sex at least five days before ovulation (or longer).

But if a woman is not having sex on those days, the 85 percent figure goes out the window.

Essentially, if a woman does not have sex five days before ovulation or on the day of ovulation, her average odds of getting pregnant in that month drop to near zero—or, as the German study found, to about 0.4 percent.

And this explains what many people find almost impossible to understand about NFP—namely, why a woman practicing NFP correctly might actually have a lower chance of getting pregnant than a woman who is using artificial birth control.

The woman using artificial birth control alone (as opposed to using artificial birth control and monitoring the physical indicators of her fertility) is not attempting to avoid sexual relations during her most fertile time of the month. Indeed, since her sex drive naturally increases around the time of ovulation, she is more likely, all other things being equal, to have sex around the day of her ovulation than the woman practicing NFP, who is actively avoiding sex at that time.

But condoms break, and diaphragms and IUDs fail, and even the Pill does not always prevent ovulation. So when you combine sex on fertile days with contraception that isn’t 100 percent reliable (and no form of contraception is), you have “surprises.” In the German study, the rate of such surprises turned out to be only slightly higher—0.6 percent versus 0.4 percent—but it is a significant increase. (Indeed, anyone who knows basic math will note that an increase from 0.4 percent to 0.6 percent is a 50 percent increase.)

That, in a nutshell, explains the findings in the German study. And it explains why many of us who practice NFP can say with all honesty that it's been completely effective for us. And, unlike artificial contraception, Natural Family Planning is effective both for postponing pregnancy and for achieving pregnancy when the time is right.

The first thing that most fertility consultants ask couples to do when they go off of artificial birth control and have trouble conceiving is to learn NFP. In many cases, a few months of charting and having sexual relations in the two days before and/or on the day of ovulation is all it takes to achieve pregnancy.

It's not rocket science—just simple biology.

None of this means, of course, that using NFP is easier than using the Pill or even a condom. It takes less willpower to use artificial contraception, and no one who uses NFP will deny that charting a woman's signs of fertility takes work. But those who attack NFP out of a mistaken belief that unprotected sex, at any point in the month, is an open invitation to pregnancy reveal nothing more than their ignorance of human biology.

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