"An app called Natural Cycles has become the first to be classified as a medical device for use as contraception. Natural Cycles https uses a womans body temperature and menstrual cycle data to tell her when she is fertile. On fertile days users must abstain from sex or use other protection such as condoms to prevent pregnancy. The app was approved as a class IIb medical device by Tv Sd a notified body in Sweden where it is based. But sexual health organisations are warning that being classed as a medical device does not guarantee that the app will effectively prevent pregnancy. Sexual health charity the FPA Fertility UK and the Faculty of Sexual and Reproductive Health of the Royal College of Obstetricians and Gynaecologists are jointly calling for more largescale independent trials to ensure that evidence for such apps is on a par with other rigorously tested methods of contraception. Natural Cycles is one of numerous apps that help women practise a method of contraception called natural family planning http also known as the fertility awareness method. Natural family planning involves monitoring your body for signs of ovulation like an increase in your temperature or a change in cervical mucus. The thing about the female egg is that it only lives for about  hours. So if you could reliably know when youve ovulated as a woman and then you give that egg two or three days to die any time after that in the cycle you will not get pregnant because theres no egg to fertilise Dr Susan Walker a senior lecturer in sexual health at Anglia Ruskin University told BuzzFeed News. Taking your temperature is what sets Natural Cycles and other apps like it apart from standard periodtracking apps. Without temperature information an app can can guess at your ovulation day based on your average cycle length but cant be sure when you actually have ovulated and so you cant use it as a reliable form of birth control. You dont actually need an app to practise natural family planning  although it does cut down on paperwork. Even so the FPA advises that natural family planning is most effective if youre taught how to do it by a specialist teacher and you monitor all three fertility indicators  temperature menstrual cycle length and cervical secretions. But natural family planning is not for everyone. For starters it can only protect against pregnancy not STIs. And if youre considering ditching the Pill because remembering to take it each day is too much effort you probably wont like having to remember to take your temperature every morning. You really have to commit to it whether youre doing it the traditional way or youre using an app. You have to take your temperature every morning and record that because without all that data its not really any use to you Bekki Burbidge head of communications at the FPA told BuzzFeed News. Its not a method thats going to suit everybody. But for women like Natural Cycles cofounder Dr Elina Berglund who want an alternative to hormonal contraception its an option worth pursuing. Along with her husband and cofounder Dr Raoul Scherwitzl Berglund created the algorithm used in the app for personal use before deciding it was something she should share with the rest of the world. When it comes to contraception its all about choice. Its not like one solution will fit all women said Berglund. Our typical user is in a stable relationship a few years before wanting to have kids. At first she starts to use the app to prevent pregnancy and when shes ready to get pregnant she switches to plan and usually she gets pregnant quite quickly because the app already knows her body and cycle. To use the app you need a monthly or yearly subscription and a thermometer that you have to use to take your temperature each morning before you get out of bed. The app then tells you if youre likely to be fertile or not that day. And if the app gives you a red day  meaning you could be fertile  you have to abstain from sex or use another method of protection such as condoms. Berglund says typically women will start with  green days  days on which its safe to have sex  on the app and that this number will go up as it learns more about them. The algorithm analyses womens body temperature and menstrual data and learns more and more from previous temperatures and previous cycles to know the unique woman she said. Natural Cycles published a paper http last year in the European Journal of Contraception and Reproductive Healthcare looking at how effective the app was at preventing pregnancy. The study involved over  users of the app. When it comes to contraceptive effectiveness youll usually get two numbers rating how effective it is with perfect use and how effective it is with typical use. Perfect use is using the method exactly as instructed every time without fail. Typical use is closer to what the average persons experience might be  for example with the Pill typical use will include people who occasionally forget to take it. Natural Cycles own analysis shows that with perfect use just . out of  people will get pregnant in a year using the app as contraception with this number rising to  in  for typical use.  When used perfectly the combined oral contraceptive pill aka the Pill results in about . pregnancies per  women per year httppatient.infodoctorcombinedoralcontraceptivepillfirstprescription. With typical use that number rises to almost  per . But the study has some drawbacks  for one it was done by the team behind the app themselves so isnt independent and the data was analysed retrospectively. According to NHS Choices http The retrospective design means data was not collected to specifically answer this question and may not be fit for purpose. And most women included were aged between  and  so theres no evidence the results would apply to women outside that range. They certainly had lots of women in this study but what they will need to do is follow women up for a longer period of time because things change Walker said. Berglund told BuzzFeed News that the Natural Cycles team analyse the data they have internally every month. She said theyre also in the process of publishing an updated version of last years study this time with around  women included in a peerreviewed journal I cant say too much about the numbers but its going in the right direction. Diana Mansour vice president for clinical quality at the Faculty for Sexual and Reproductive Healthcare said in a statement There are hundreds of apps which claim to help users plan or prevent a pregnancy  these come in all manner of forms and some will be more effective than others. Without independent evaluation we cant say for certain which will be effective at achieving this. Women who wish to use fertility awarenessbased contraception are advised to receive guidance from a qualified teacher to learn how to effectively monitor the different indicators. Apps currently do not come with this teaching leaving room for misunderstanding and inaccurate use. If youre thinking about changing your method of contraception try the FPAs My Contraception Tool http or speak to your GP."