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NFP and Effectiveness

Effectiveness has in then past been seen as a big issue for Natural Family Planning. After decades of research consistently returning positive results, one might hope that this had been overcome, but unfortunately the message seems slow to spread - possibly not helped by inaccurate reporting in some quarters with vested interests in the contraceptive industry.

This document seeks to lay out both the research findings which are available and the meaning of the terms often used, so that a greater understanidng can be brought to bear on this topic.

Contents:

  1. Introduction
  2. Measurements of Effectiveness using the Pearl Index
  3. Measurements of Effectiveness using Life Table Calculations
  4. Conclusion
  5. References

The Effectiveness of Natural Family Planning

Introduction

One of the most frequent challenges faced by proponents of Natural Family Planning methods is over effectiveness, and there can be no doubt that this arises from the poor image of the very early forms of NFP - the Calendar Rhythm method of the 1930's and 1940's. Despite the fact that these methods were superseded fifty years ago, they are still referred to by some references dealing with the effectiveness of family planning methods in general - thus one may see a leaflet providing 1990's effectiveness figures for, say, the Pill alongside 1940's effectiveness levels for the Rhythm Method.

In reality, modern Natural Family Planning methods perform extremely respectably against other forms of family planning. Effectiveness levels are 99% or higher, comparing with 99%+ for the Pill and 97% for the Condom.

Understanding the figures for effectiveness of family planning methods is itself something of a science. There are two measures of effectiveness: the Pearl Index and the Life Table Calculation.

Measurements of Effectiveness using the Pearl Index

The most commonly quoted measure of effectiveness is the Pearl Index, which is defined as the number of unintended pregnancies per hundred women per year - that is, the number of pregnancies in 1200 observed months of use.

This measure has severe drawbacks. It assumes, for example, that the effectiveness rate of a method of family planning is constant over time, whereas in fact all methods increase in effectiveness as a couple uses them for longer. This, together with other limitations, makes it an index which can be difficult to compare between different studies with their different methodologies, and between different methods of family planning which will be affected by other factors - such as dissatisfaction, health side effects etc. - which will not be accounted for in this treatment of effectiveness. In addition, some studies have used 1300 cycles of experience rather than 1200 months as their base.

Pearl Indices are often converted to percentage effectiveness rates. In the condom example above, therefore, the effectiveness rate would usually be stated as 97%.

Despite these problems, Pearl Index rates are often seen, perhaps because they are simple to explain, and perhaps because the figures they produce are lower than the alternative Life Cycle Calculations and thus provide a greater appeal to proponents of any family planning method. A condom manufacturer, for example, would rather quote a Pearl "failure" level of 3% than a Life Cycle calculation "failure" rate of 9.8%.

Two tables are provided below: Table 1 is for the studies of Natural Family Planning effectiveness carried out since 1980, providing the Pearl Index found in each study. Table 2 gives the Pearl index for various methods of family planning, for comparison purposes.

1. Pearl Indices in Studies of Symptothermal NFP
Study LocationYearPearl IndexRef.
Germany2007 (0.4), 1.8 12
Europe 1992 2.8 1
Germany 1991 2.3 2
United Kingdom 1991 2.7 3
Italy 1986 3.7 4
India 1982 0.2 5
USA, France, Mauritius, Canada, Colombia 1981 (0.9) 6
USA 1981 (0) 7

In the above table, the figures in brackets are those for 'perfect use' - i.e., for the use of the method strictly according to its rules, with any unintended pregnancies arising from misunderstanding or misinterpretation excluded. The remaining figures are 'actual use' figures, representing the overall effectiveness in the survey, regardless of such problems.

In the light of the above figures, one can see that the Symptothermal Method of NFP (STM) is a highly effective method of family planning. Clearly, there are variations in the figures, and the studies have used differing NFP methodologies - different 'rules' for the identification of the fertile time, and improving methodologies are learning from the results of past surveys.

Taking the 'average' of these figures yields a Pearl Index in the region of 2.8 for actual use and 0.3 for 'perfect' use Now we can put this in the context of other family planning methods:

2. Pearl Indices for Various Family Planning Methods
MethodPearl Index - 'Perfect Use'Pearl Index - 'Actual Use'
Pill 0.1-0.5* 3
Natural Family Planning (NFP) 0.3 2.8
Intra Uterine Device (IUD) 1.5 2
Male Condom 3 12
Female Condom 5 21
Diaphragm + Spermicide 6 18
Withdrawal 4 19
Spermicide Alone 6 21

The data here (other than that for NFP, for which see above) are drawn from "Contraceptive Technology" (8), a standard reference text on the subject, quoted by most agencies. Unfortunately, this provides a Pearl Index for 'Periodic Abstinence' which lumps together all possible methods of NFP, including those which were outdated by the mid 1940's - which may be of academic interest but is hardly practical in today's world.

*The two 'perfect use' figures for the Pill are for the combined and progestin-only versions respectively: this is the only place in which the two types are separated in effectiveness tables.


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Measurements of Effectiveness using Life Table Calculations

Life Table calculations are far more complex to explain and to undertake. They take into account information on a variety of factors influencing those taking part in studies, their continuation with the method etc. This data is used to calculate a separate effectiveness rate for each month of the study, feeding through to an overall probability of conception over a specified timescale (usually 12 months). This is a substantially different figure from the Pearl Index, but in terms of comparability between methods it has considerable advantages.

Again, two tables are provided below: Table 3 is for the studies of Natural Family Planning effectiveness carried out since 1980, providing Life Table rates. Table 4 gives Life Table rates for various methods of family planning, for comparison purposes.

3. Life Table Rates in Studies of Symptothermal NFP
Study Location Year Life Table Rate Ref.
Zambia*1990 8.9 9
Liberia*1990 4.3 9
USA 1981 11.2 (0) 7
USA 1986 14.4 10
USA, France, Mauritius, Canada, Colombia 1981 7.2 6

In the above table, the figure in brackets is those for 'perfect use' - i.e., for the use of the method strictly according to its rules, with any unintended pregnancies arising from misunderstanding or misinterpretation excluded. The remaining figures are 'actual use' figures, representing the overall effectiveness in the survey, regardless of such problems. The two surveys marked with a (*) included couples using the Billings (mucus-only) method and did not differentiate between methods in the published results.

Again, one can see that the STM is a highly effective method of family planning, yielding a Life Table Calculation in the region of 4-14% for actual use Now we can put this in the context of other family planning methods:

4. Life Table Rates for Various Family Planning Methods
Method Life Table Calculation Range(%)
Intra Uterine Device (IUD) 3 - 5
Pill 4 - 9
Natural Family Planning (NFP) 4 - 14
Condom 10 - 18
Diaphragm + Spermicide 12 - 39
Withdrawal 14 - 27
Spermicide Alone 21 - 25

The data here (other than that for NFP, for which see above) are drawn from data from the Guttmacher Institute, an affiliate of Planned Parenthood (11). This is a standard reference text on the subject, quoted by most agencies.


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Conclusion

In a variety of studies over the last 15 years, NFP has been shown to provide a high level of effectiveness among populations of many countries and cultures. Placing these results - whether measured by Pearl Index calculations or by Life Table methods - alongside other family planning methods leaves NFP ahead of the majority of methods, and behind only the Pill and the IUD, both of which have abortifacient mechanisms (for more detail on this see this part of 'Contraception - Why not? on this website). For those motivated primarily by effectiveness, but who respect human life from its conception and/or wish to avoid pharmacological side effects, Natural Family Planning should be the method of choice.



References

1. Prospective European Multi-Centre Study of Natural Family Planning (1989-1992): interim results. Adv Contracept 9:269-283 1993
2. Frank-Herrmann, Freundl, Baur et al, Effectiveness and Acceptability of the Symptothermal Method of NFP in Germany, Am J Obstet Gynecol 165:2052-2054. 1991
3. Clubb, Piper, Knight. A Pilot Study on Teaching NFP in General Practice. Proceedings of the Conference at Georgetown University, Washington DC 1991
4. Barbato M, Bertolotti G, Natural Methods for Fertility Control - A Prospective Study. Int J Fertil 48-51, 1988

NB:References 2,3 and 4 above are available on-line at the "Fertility" site - look for 'Newsletter 17'

5. Ghosh, Saha, Chatterjee, Symptothermia vis-a-vis Fertility Control, J Obstet Gynecol India 32:443-447, 1982

Return to: Table 1 Table 2 Table 3 Table 4


6. Rice, Lanctot, Garcia-Devesa, Effectiveness of the Symptothermal Method of NFP: an International Study. Int J Fertil 26:222-230, 1981
7. Wade, McCarthy, Braunstein et al, A randomised prospective study of the use-effectiveness of two methods of NFP, Am J Obstet Gynecol 141:368-376, 1981
8. Hatcher, Trussell, Stewart et al, Contraceptive Technology: 16th Edition Irvington Publishers, New York, 1994
9. Gray, Kambic, Lanctot et al, Evaluation of NFP Programmes in Liberia and Zambia J Biosoc Sci 25:249-258 1993
10. Kambic, Kambic, Brixius, A 30-month Clinical Experience in NFP, Am J Pub Health 71:1255-1257, 1981
11. Choice of Contraceptives, Med Let 34:111-114, 1992
12. P. Frank-Herrmann et al, The effectiveness of a fertility awareness based method to avoid pregnancy in relation to a couple's sexual behaviour during the fertile time: a prospective longitudinal study Hum. Reprod., Advance Access published on February 20, 2007


Return to: Table 1 Table 2 Table 3 Table 4




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