I wrote the letter below to BMJ Sexual and Reproductive Health in response to the article by Amy Hough et al "Social media and advertising natural contraception to young women: the case for clarity and transparency with reference to the example of Natural Cycles" (paywall). The BMJ allows publishing of paywall content on author websites so my letter is below. To see it on the BMJ SRH website click here (paywall).
A Cochrane Review is an understandable choice but not an appropriate one. It would be more helpful to quote NHS Choices, the Family Planning Association or Faculty Guidance, which all have a more nuanced approach to the evidence, and say that fertility awareness is up to 99% effective.
Typical use rates are lower but many women achieve high effectiveness thanks to either their own research or appropriate support.