Varuna Srinivasan, MBBS, MPH, Reagan Beyer: National Center for Health Research.
There are several apps currently available to help a woman track her fertility in the hopes of getting pregnant. These include Kindara, Glow, Groove, Fertility Friend, Clue, Conceivable and Ovia – all help a woman track the days when she is most fertile. Ranging from $0.99 for the app to $45/month, each app provides a unique service of tracking fertile days, using a basal body temperature chart, ovulation calculator, menstrual calculator or simply serving as a fertility planning journal. 
Of course, if you can track the days you are most fertile, the same information can be used to try to prevent pregnancy. On August 10, 2018, the FDA approved Natural Cycles as a contraceptive tool.  This Swedish-based app, which was already available in Europe, costs $75 per year and comes with a thermometer. This app uses a statistical algorithm that calculates fertility from user-entered information, including basal body temperature, and assigns days as “red” or “green.” “Red” days represent days on which it is unsafe to have sex if a woman wants to avoid getting pregnant. Women are advised to use other forms of contraception, such as condoms, pills or abstinence, on these days. “Green” days are considered “safe” days on which women can have unprotected sex with their partners. 
Natural Cycles claims on their website that the reliability of the application is comparable to that of a condom and has a method failure rate of 7%, meaning that 7 women out of the 100 women using the app will get pregnant in any year it is used. 
That sounds effective compared to, for example, condoms (which has a method failure rate of 18%). Fertility awareness methods, or ‘Calendar’ Methods, that rely on a combination of basal body temperature recordings and cervical mucus based on a woman’s menstrual calendar, are known to have a method failure rate of 25%. 
But is the 7% method failure rate listed for Natural Cycles accurate? There have been reports of more than 3 dozen women in Sweden getting pregnant while using this app. More on this can be found here.
In 2018, Natural Cycles was banned from being advertised in the UK due to false claims of efficacy, because the app advertised itself to over 100,000 UK users as being highly accurate in preventing pregnancy and as a form of “clinically tested alternative to birth control.” More on the reason for this ban in the UK can be found here. They point out that 7 women out of 100 will become pregnant during a year of use if the app is inaccurate, they have unprotected sex, or do not use contraception on a red day. Even if used perfectly, an additional 1 woman out of every 100 would become pregnant after having unprotected sex on a falsely-attributed green day, or if contraceptive methods failed during intercourse on a red day. This is not as effective at preventing pregnancy as the correct use of most hormonal contraceptives.
While fertility apps that are aimed to help you get pregnant are not entirely accurate either, they help women keep track of their last menstrual periods, fertile days, and as a journal to keep track of important fertility tests. For these, the benefits of using the app far outweigh the risks. That does not seem to be true of Natural Cycles use to prevent pregnancy.
Clue is a period tracking app that received FDA clearance in 2021 to also be used as a digital birth control device.  The standards for clearance are not as rigorous as the standards for FDA approval, and as of April 2021, the app has not yet launched the birth control feature. The company is expected to launch the feature later in 2021. Clue will not use basal body temperature as part of its algorithm, although it was cleared by the FDA based on the conclusion that it is “substantially equivalent” to Natural Cycles, which is based on basal body temperature.
So how does Clue work? Clue bases fertility predictions on a technology, Dynamic Optimal Timing (DOT), that was tested in a large clinical trial of 718 women.  The study claimed failure rates of only 1% with perfect app compliance and about 6% for typical-use.  Typical use includes the possibility of human entry error and having unprotected sex during “high risk days.” Is 94% accuracy good enough for most women? Pregnancy can be physically dangerous as well as having psychological and economic consequences for women or families who try to avoid it. For that reason, Clue might be a reasonable choice only for women not able or willing to use IUDs, birth control pills, diaphragms, or condoms.
You can read more about the FDA clearance for Clue here.
Dr. Zuckerman spoke to The Verge regarding the FDA clearance for Clue. You can read more here.
The Bottom Line
All fertility apps are heavily dependent on users putting accurate information into the system. Women using any of these apps would need to be diligent in entering their information accurately and on time every day. Our advice and recommendation is that whether you are trying to conceive or trying to avoid pregnancy, consult with your gynecologist to find the right tools to help you achieve your goal. Natural Cycles and Clue are not reasonable alternatives to more proven forms of contraception, but could help women not willing to use IUDs, birth control pills, diaphragms, or condoms.
- Healthy Women, Fertility Apps: Which One Is Best for You? www.healthywomen.orghttps://www.healthywomen.org/content/article/fertility-apps-which-one-best-you
- Food and Drug Administration, DE NOVO CLASSIFICATION REQUEST FOR NATURAL CYCLES, www.fda.gov,https://www.accessdata.fda.gov/cdrh_docs/reviews/DEN170052.pdf
- Natural Cycles, Digital Birth Control, www.naturalcycles.com,https://www.naturalcycles.com/en-us/birthcontrol/howitworks/
- Food and Drug Administration, Birth Control, www.fda.gov, https://www.fda.gov/forconsumers/byaudience/forwomen/freepublications/ucm313215.htm
- The Verge, Clue gets FDA clearance for digital birth control, https://www.theverge.com/2021/3/1/22307991/clue-digital-birth-control-natural-cycles-fda
- TechCrunch, Clue gets FDA clearance to launch a digital contraceptive, https://techcrunch.com/2021/03/01/clue-gets-fda-clearance-to-launch-a-digital-contraceptive/#:~:text=The%20Berlin%2Dbased%20company%20is,a%20US%20launch%20in%202021
- Jennings V, Haile LT, et al. Perfect- and typical-use effectiveness of the Dot fertility app over 13 cycles: results from a prospective contraceptive effectiveness trial, The European Journal of Contraception & Reproductive Health Care, 24(2):148-153. https://doi.org/10.1080/13625187.2019.1581164