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Is the NaturalCycles app just too good to be true?

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A new study has been making the headlines which claims that a fertility tracking app is as effective as the hormonal contraceptive pill for prevention of pregnancy. But although these results sound exciting, is it really time for women to ditch the pill?

Fertility-tracking apps are on the rise – a quick scan of the Google Play Store reveals around 30 apps designed to monitor and predict fertile windows in a woman’s reproductive cycle. NaturalCycles is one such app, developed by Swedish couple Elina and Raoul Scherwitzl, and is the focus of a recent study published in the European Journal of Contraception and Reproductive Healthcare.

The researchers measured the effectiveness of the app in preventing pregnancy by calculating its Pearl index value, a measurement that enables comparison of the efficacy of different forms of birth control. The higher the Pearl index, the greater the chances of an accidental pregnancy.

The app allocates ‘green’ and ‘red’ days that indicate when it is safe to have unprotected sex

many colored condoms on white background

These probably won’t be going out of fashion anytime soon. Image: Hurrikán Hurrikán / picasaweb

The researchers calculated that the NaturalCycles app has a Pearl index of 0.5, which means that if 1000 women use the app correctly over the period of one year, it will result in five pregnancies. The Pearl index of the hormonal contraceptive pill ranges from 0.1–1.

Like many of its counterparts, NaturalCycles works on the basis that ovulation is followed by a rise in basal body temperature, generally between 0.1–0.5 degrees Celsius, that lasts for up to 24 hours. Women who use the app are required to input accurate daily temperature measurements, as well as the dates of their menstrual cycles.

The app uses an algorithm with this information that allocates ‘green’ (safe) and ‘red’ (unsafe) days that indicate when it is safe to have unprotected sex. New users initially begin with more red days, but the number decreases as the app begins to learn your patterns.

A third author is an employee of NaturalCycles. This is a clear conflict of interest

The results from this initial study sound promising and may appeal to women who desire a hormone-free alternative to the contraceptive pill. Yet there are some limitations.

Firstly, two of the four study authors are the founders of NaturalCycles and have a stock ownership in the company. Given that the app costs users £7 a month to use, and with data analysed from 4054 women, there are significant financial gains to be made from it. A third author is an employee of NaturalCycles. This is a clear conflict of interest.

Furthermore, the analysis was conducted retrospectively, which means that it uses data that was not specifically collected for the purposes of research, and may therefore be subject to user-bias or other factors that have not been considered. If a woman decided to use a condom on a green day, for example, this would artificially improve the app’s performance.

For now, I’m not sure it’s worth taking your chances on

The Pearl index is a ‘perfect-use’ index; this assumes that all women using the app are using it correctly 100% of the time. Taking accurate basal body temperature measurements is not trivial: temperature must be taken immediately upon waking, after a period of at least four hours of uninterrupted sleep. In contrast, the pill merely requires swallowing a tablet every day – something that is less prone to variation.

For a more accurate assessment of the app’s effectiveness, a prospective study over a longer time frame needs to be conducted, ideally by independent researchers.

For now, I’m not sure it’s worth taking your chances on.

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