The Effectiveness of Natural Family Planning
The effectiveness of any family planning method is a percentage derived from studies that have determined surprise pregnancy rates. A surprise pregnancy rate is measured in terms of the number of couples out of 100 who become pregnant using a method for one year. Therefore, a surprise pregnancy rate of 15% or 0.15 means that statistically 15 couples out of 100 might conceive during one year of using that particular method of birth control.
A 99% effective rule or method like NFP means that statistically 99 couples out of 100 would not conceive during one year of using that particular method of family planning.
The method effectiveness of a particular method of family planning assumes its “perfect use.” As effectiveness rates are calculated by the number of surprise pregnancies, method effectiveness calculations only include those pregnancies that resulted from correctly following the rules of the method and the months or cycles in which the method was correctly and consistently used.
For Natural Family Planning, this means that the couples correctly followed and applies all of the rules.
A parallel example of method effectiveness involving the birth control pill would assume that the Pill is taken exactly as prescribed at the same time every day without exception. Another example involving condoms would mean that a condom was used correctly and consistently every single time.
User effectiveness of a particular method of family planning is based on the actual practices of the couples using the method. It refers to all pregnancies occurring during a study and all months or cycles, including incorrect and correct application of a method and its rules.
So if a couple decide to ignore signs of fertility, or completely misunderstand the method, and become pregnant as a result, that affects the method's user effectiveness measure.
These terms are applicable to both contraception and NFP. If a couple become prgnant becasue they didn't use a condom correctly, that affects the condom's user effectiveness measure.
2007 German Study of STM Effectiveness
A major German study of the effectiveness of the sympto-thermal rules associated with NFP was published in 2007. The study used rules that were only slightly different from those taught by CCL, but were close enough to provide a valid comparison to the CCL method.
This study, published in the journal Human Reproduction, found a method effectiveness of 99.6% and a user effectiveness of 98.2%.
Comparing Effectiveness Rates
This graph compares the method effectiveness of various forms of contraception with NFP. The Sympto-Thermal Method (STM) of NFP taught by CCL in this course is extremely effective in determining the fertile and infertile times within a woman’s cycle. The 2007 German study also showed that couples who have a desire to postpone a pregnancy during a given cycle, can do so using NFP with the same accuracy as with contraception.