The Science Behind NFP

Science Basics

How Are Effectiveness Rates Determined?

This can be a complicated topic because Effectiveness Rates for Natural Family Planning can include both the effectiveness of avoiding pregnancy and the effectiveness of achieving pregnancy. Furthermore, not all NFP methods include the effectiveness rate for achieving pregnancy nor do they all classify pregnancies during the studies the same. To complicate matters even more, some methods of NFP exclude participants from their studies who do not have "regular cycles". If all of those factors weren't enough, there is one more. Studies have shown that the effectiveness rate of NFP users can vary based on how serious they are at avoiding pregnancy. A couple who is avoiding pregnancy but intends to get pregnant in one to three more cycles is far more likely to have an unintended pregnancy using NFP than a couple who has a serious medical reason that could result in death. So in order to look at NFP effectiveness rates, it is important to take all of these things into consideration. 

The typical way a contraceptive is studied for effectiveness is by use of something called the "Pearl Index". The Pearl Index classifies effectiveness rates by looking at the number of pregnancies per 100 users over the course of 1 year.  At first glance, this seems like a very reasonable way to assess the effectiveness rate of avoiding pregnancy. However, when you look more deeply at it, NFP users can (and do) change their intention for using NFP from avoiding pregnancy to achieving pregnancy, so if you calculate every pregnancy that occurs against the effectiveness rate for avoiding pregnancy, then it inaccurately skews that method's effectiveness rate for avoiding. Put another way, if a couple intentionally switches from avoiding pregnancy to achieving pregnancy and does, in fact, get pregnant, then that pregnancy cannot be classified as "unintended" and should not be included in the effectiveness rate for avoiding pregnancy.  Because many researchers try to apply the Pearl Index criteria to NFP and ignore the factors mentioned above, it is easy to find research on the Internet that has NFP effectiveness rates for avoiding pregnancy as quite low. To help alleviate this confusion, this section will discuss research that assesses multiple factors of NFP use. All research that seeks to assess NFP in the way contraceptives are assessed will not be included. To help make it easy for you to read, the factors used in the studies will be explained. Effectiveness Rates for avoiding pregnancy will be expressed in Actual Use Effectiveness and not Theoretical Use Effectiveness

Before we dive into the actual studies; however, it is important to remember that new NFP research is constantly coming out and NFP methods are updating their methods to continually improve their effectiveness rates. While the research on this page is up to date as of this writing, it may be outdated sometime soon. Below is information on each of the methods we have in the Diocese of Lansing (in alphabetical order) and some quick ways to assess how their effectiveness rates were calculated.

Billings Ovulation Method

The Billings Ovulation Method was one of the first methods of NFP developed after the release of Humanae Vitae in 1968.  

The Creighton Model FertilityCare System

The Creighton Model has been studied by multiple researchers including Dr. Richard Fehring, developer of the Marquette Method.

Key Findings:

Avoiding Pregnancy: 96.8 - 98%.

Achieving Pregnancy: 22 - 24.4%

Couples switched intention: 

Key Factors:

Unintended pregnancy was calculated as any pregnancy in which the couple believed that they were not fertile and could not get pregnant on the day of intercourse that resulted in pregnancy.



Is this Study Legit?

Peer-Reviewed Research