Well... it depends. If you only rely on this method for a few days each month immediately after the end of menses, the chances of it failing are MUCH lower tha

Well... it depends. If you only rely on this method for a few days each month immediately after the end of menses, the chances of it failing are MUCH lower than if you rely on it for a week prior and a week after menses. That said, wiki claims[1] that the latter approach is >95% effective in perfect use, and 88% effective in typical use. Compare this to withdrawal[2], which wiki claims is 96%/78% effective respectively. [1]: https://en.wikipedia.org/wiki/Calendar-based_contraceptive_methods#Standard_Days_Method https://en.wikipedia.org/wiki/Calendar-based_contraceptive_m... [2]: https://en.wikipedia.org/wiki/Coitus_interruptus https://en.wikipedia.org/wiki/Coitus_interruptus