The Rhythm Method was based on counting days… but this was problematic for many because not every woman has the same “rhythm”… in fact, even for the same woman it’s extremely rare to find that maintains the same “rhythm” from month to month!
Hence the change to Natural Family Planning… these are *observation *methods. They allow the woman to track her daily fertility symptoms and make judgments based on real data, rather than just counting days.
There are several different methods of NFP… including Sympto-Thermal, Creighton, Marquette, Billings, etc… most involve tracking a woman’s cervical mucous, and some add additional signs such as basal body temperature, cervical position, and possibly using fertility monitors.
The rhythm method is a calendar method of nfp–based on the average woman’s cycle length and the average timing of ovulation. If a woman’s cycle is long or short, this will throw the method off and pregnancy is more likely to occur.
Today, there are various forms of nfp that are based on a woman’s physical signs of fertility–basal body temperature, cervical mucous, and cervical position and some add a fertlity monitor like Clear Blue Easy
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