There are several different methods of NFP (natural family planning). All of them have good effectiveness if used correctly. Here is a page with a summary of the different methods of NFP.
The effectiveness of any method of NFP varies from one couple to another.
All NFP methods are significantly less effective for couples who do not follow the method closely.
Older couples are generally less fertile than younger couples.
All NFP effectiveness percentages refer to couples who practice the method correctly and strictly. Different studies show different degrees of effectiveness; there is no single exact number to cite. A 95% method effectiveness would mean that 5 out of 100 women, practicing the method correctly for one year, will get pregnant. Individual effectiveness varies.
The Sympto-Thermal Method: ~99% effective
The Creighton Method: >98% effective
The Billings Method: >97% effective
Two-Day Method: ~96%
Standard Days Method: ~95%
Ecological Breast-Feeding (LAM): ~94% effective
The Standard Days Method is perhaps the easiest method of NFP to learn. It was developed at Georgetown University, as an update of the older calendar method. Its effectiveness is 95% with correct use, meaning that of 100 women who use the method in year, 5 will become pregnant.
Here is a page describing how to use the Standard Days Method. PDF format brochures are available from a link on the same page, at the upper right top of the page.
SDM works for women with menstrual cycles from 26 to 32 days long. To use the method, couples abstain from sexual intercourse on days 8 through 19 of the woman’s menstrual cycle. If a woman has more than one cycle per year that is shorter than 26 days, or longer than 32 days, the method effectiveness decreases significantly and a different method of NFP should be used.
The disadvantages of SDM are that the couple seeking to avoid pregnancy must refrain from sexual relations for 12 days out of each cycle, and that the method is a little less effective than other methods.
The advantages of SDM are that it is very easy and quick to learn. It is not necessary to take a class. The method can be learned online or from a book. You do not need to use a special thermometer, nor is there any complex charting.
If you have any questions, please add them in the comments.