No method of family planning is 100% effective in avoiding pregnancy other than … 

No method of fertility regulation is 100% effective in avoiding pregnancy other than abstinence, and castration. Unintended pregnancies occur no matter what method of fertility regulation is used whether natural or artificial.

NFP (Natural Family Planning) is the only method of fertility regulation that can be used both to achieve pregnancy as well as to avoid pregnancy.

One of the couple’s main considerations when choosing a particular family planning method is the effectiveness of that particular method in avoiding pregnancy.

What methods are used to measure effectiveness in avoiding pregnancy: 

The measure of effectiveness in avoiding pregnancy that is often quoted is the Pearl Index. The Pearl Index is defined as the number of unintended pregnancies per hundred women per year while using the particular NFP Method –  that is the number of pregnancies in 1200 observed months of use. (1)

Effectiveness is classified:

The effectiveness of a family planning method is classified under the headings: (a) Method Failure and (b) User Failure.

Method failure

Method failure is when an unintended pregnancy occurs when using the method correctly, and in the case of natural family planning when adhering to the rules of the NFP Method.

User failure

User failure is when an unintended pregnancy occurs if the rules or instructions for the particular method have not been strictly adhered to; e.g. if pills have been missed when using oral contraceptives, or in the case of natural family planning, not complying with the rules of the NFP Method. The ‘user failure’ rate in natural family planning depends very much on the couple because this is where factors such as not adhering to the rules, or not fully understanding the method have an influence and it underlines the importance of good teaching by a qualified Natural Family Planning teacher.

Some critics of the Pearl Index suggest that other methods such as Life Table Analysis (i.e.the Kaplan-Meier method), or the Perfect / Imperfect Use Model of Grummer-Strawn, give a more accurate picture of Natural Family Planning effectiveness.(2)

  1. Freundl G, ‘Prospective European multi-center study of natural family planning (1989-1992): interim results’ p277; Advances in contraception; 1993; 9;269-283.
  2. Frank-Hermann, Petra, Gnoth, C, Baur S et al ‘Determination of the fertile window: Reproductive competence of women- European cycle databases’; Gynecological Endocrinology, p307,308; June 2005, Vol 20, issue 6, p305-312.