Clarity, I think you are correct that many NFP and FAM couples mix the two methods ---albeit unknowingly---but by their definition, NFP promotes abstinence during the fertile phase of a woman’s cycle.
NFP is the Sympto-thermal method(STM), meaning that it charts a woman’s fertility sign ( primarily cervical fluid/mucus, but also the cervical position) and crosschecks those signs with the basal, or waking, temperature. The opening of the cervix, the os, will give a 1-2 day lead time on changes in cervical fluid. FAM also instructs how to gauge the fertility signs. I am not aware of any research showing that FAM is more effective than NFP. The failure rates for normal condom use would indicate the opposite. (The Billings folks, the Families of the America’s folks and the CCLI folks provide efficacy data to those interested). The MAIN difference between NFP and FAM is that NFP promotes abstinence during the fertile period, called Phase II. Another difference is that the world’s major religions accept NFP. The Billings method is ovulation-only and has been successfully used all over the world, promoted by the WHO. Interesting note: Mother Teresa taught natural family planning to her nurses in her hospitals and those who followed her instructions were excluded from India’s mandatory sterilization program. How about that for effectiveness : ) FYI, there is yet another method, the Creighton model.
While any natural method is easier with a *regular cycle*, very few women have a 28 day cycle every month, year after year. Most women observe some fluctuation in overall cycle length of 2-5 days; however, their luteal phases remain fairly constant each month. Broadly speaking, the luteal phase is the time from ovulation and the onset of menses. Charting provides women with a detailed look at what’s going on in their cycle, regardless of the method.
Hope this helps : )