NFP is NOT the Rhythm Method.
The Rhythm (or Calendar) method was developed in the 1930s. It was based on the theory that the time of ovulation could be determined by calculating previous menstrual cycles. This method often proved inaccurate because of the unique nature of each woman’s menstrual cycle: some women have very irregular cycles and almost all women have a cycle of unusual length once in a while.
NFP in Our Times
Modern NFP methods, however, are progressive. That is, they are based on progressive, day-to-day observations of the naturally-occurring signs and symptoms of the fertile and infertile phases of the menstrual cycle. NFP methods take advantage of the changes associated with ovulation, treating each cycle as unique.
Study after study has shown NFP to be up to 99 percent effective at helping couples avoid pregnancy. After a large scale, multi-year study comparing NFP to the IUD, for example, the Chinese government was so impressed with NFP that it officially declared in 1995 that the method must be presented to all citizens as part of its compulsory family planning instruction. There are currently more than 4 million couples using NFP in China to avoid pregnancy with an overall success rate of 98+ percent.
In early 2007, a large-scale German study also confirmed that NFP is highly effective. Researchers studied cycles of 900 women and found that couples who used the sympto-thermal method and abstained during the fertile time had a pregnancy rate of less than one percent.