• natural family planning

If you are a young female in the UK that uses social media (as 96% of 16–24-year-olds do),1 you may have come across an advert for Natural Cycles since they began their media campaign in late 2016, when using platforms such as Facebook, Instagram or Twitter. Natural Cycles is a ‘natural contraception’ app that was created by a Swedish particle physicist and her partner. For £39.99 per year, users log their body temperature each morning, and occasionally other details, such as their menstruation, and an algorithm determines their fertile days (during which the user would need to use barrier contraception or avoid sexual intercourse). It has been approved as a medical device for contraceptive purposes in Europe.2

The most recent study of the app was funded and primarily run by Natural Cycles. The study was a prospective observational study of 22 785 users who were mostly Swedish and had an average age of 29 (SD=5) years. They do not mention the previous contraceptive practices of their population. Women were considered pregnant if they reported it to the app, or if the timing of when they stopped using the app made it likely they were pregnant. With perfect use, Natural Cycles claims a failure rate of only 1.0 pregnancy per 100 woman-years. With typical use of the app, the failure rate increases from 1.0 to 6.8 pregnancies per 100 woman-years. However, less than 10% of the user data qualified as ‘perfect use’ and contributed to the analysis. If they included all women for which the pregnancy status was unknown (who could possibly be pregnant) this went up to …