Rhythm method for the app generation?

There is no guaranteed method of preventing pregnancy; the combined pill, when used correctly, is over 99% effective and condoms 98%. So when Jasmine saw a claim from fertility smartphone app ‘Natural Cycles’that it was “99.9% safe”, she was naturally sceptical and decided to see what evidence the developers had to support their claim.

Natural Cycles uses resting body temperature readings to determine a woman’s menstrual cycle – what’s know as a fertility awareness method. Users input this data into the app which calculates the days when it is safe to have sex. How could this process be 99.9% safe?

Natural Cycles sent Jasmine a scientific paper, and we put her in touch with Dr Sharon Cameron, a consultant gynaecologist and Co-Director of the Clinical Effectiveness Unit, Faculty of Sexual and Reproductive Healthcare, to help read it.

The paper Jasmine was sent suggested that, in theory, if this method is used perfectly (i.e. if the exact right days are chosen to have sex), the safety rate would indeed be about 99.9%. But that is assuming perfect knowledge and perfect use, which is far from a real world scenario. Lots of things can alter body temperature beyond the menstrual cycle, including infections, some medications, even just exercise. Also, the method requires cycles to be very regular, and so wouldn’t be suitable for many women anyway.

In fact, the typical failure rate of (single indicator) fertility awareness methods is as high as 24%, and this app won’t change that.

So: 99.9% effective? Only if used 100% accurately. Which it won’t be.


This ask for evidence story was written by Chris Worsfold, a volunteer at Sense About Science

 Image by Jenny Lee Silver - (CC BY-NC 2.0)



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