Contrary to what we are usually led to believe from an early age, pregnancy is only possible for a finite amount of time each cycle.
A man is of course fertile all the time, so it is the combined fertility which has to be taken into account.
FABM (Fertility Awareness Based Methods) of contraception cover a range of methods taught by many institutions to do this.
Natural She teaches the Sympto-Thermal Method, as taught by the NFPTA (the Natural Family Planning Teachers Association) UK, which uses both basal body temperature and cervical mucus observations as the two primary fertility indicators, alongside cervical position and previous cycle history, to determine when the fertile phase of a menstrual cycle begins and ends.
This insight can be used as a natural contraceptive or to help conception. If trying to avoid pregnancy then the most effective way is to abstain from intercourse during your fertile phase and this is what the NFPTA teaches. Once you know how to identify which days are fertile, you can of course do what you like with the information. Other combinations exist, such as using a barrier method or withdrawal during your fertile days (although this will bring the effectiveness down a little).
If you don't yet have full confidence in your charts or the method, you could also use the information to abstain during your fertile days and use another method in your infertile phase (safe days).
This is completely different from what is more commonly known as 'The Rhythm Method', which only takes into account previous cycle lengths and assumes that we all ovulate on day 14. The Rhythm Method completely ignores the activity happening in your cycle at the time in question and gives natural birth control a bad name as, although by definition it is natural, it is not accurate. The Sympto-Thermal Method is the most effective of the FABMs, and has been studied (Frank-Hermann et al 2007), showing a pearl index efficacy of 99.6% success rate when used as the sole form of contraception. The same study indicated that, if used in conjunction with another method during the fertile time, the rate comes down to 99.4% (with condoms) and 98.8% (with coitus interruptus (withdrawal/pull out)). As well as giving the lowest rate of unintended pregnancy of all the natural birth control methods, it also, potentially, gives the largest window of time where couples don't need to stay protected or abstain.
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