Natural Cycles Fertility Awareness App: Health Information

What is it?

Natural Cycles is the first smartphone application (“app”) for contraception cleared by the FDA. Visit our advocacy fact sheet to learn more about its expedited clearance process and potential privacy concerns.

Natural Cycles is a high-tech version of the classic rhythm method (also called natural family planning or the fertility awareness method) in which users track their ovulation cycles in order to avoid pregnancy. It is one of several dozen fertility apps available for download in the US that uses personal health information uploaded by users to predict days on which it is and isn’t safe to have unprotected sex. It is marketed as a new, technology-based, and “100% natural” form of contraception. Natural Cycles does not prevent against sexually transmitted infections.


How does it work?

Natural Cycles combines data about your menstrual cycle and your daily basal body temperature to predict when you’re most likely to get pregnant.

Your basal body temperature (BBT) is your baseline temperature when you’re completely at rest. It is measured by a special, more sensitive thermometer and should be your lowest temperature in a 24-hour period. BBTs are used in fertility tracking because there’s a slight increase in body temperature around ovulation. But accurately recording your BBT is notoriously difficult because the temperature reading will change if:

  • You don’t take it immediately after waking up
  • You get out of bed before taking it, for example, to use the bathroom, get a glass of water, or because your thermometer isn’t within reach
  • You are sick or hungover
  • You sleep less or more than usual
  • You are traveling, especially to a different climate

The company stores data from your BBT readings and past cycles in its database. Then for a subscription fee, it crunches the numbers using a proprietary algorithm to make predictions about when you’re most likely to become pregnant. The app displays either a green “infertile” reading letting you know it’s safe to have unprotected sex or a red “fertile” reading warning you to abstain from sex or use a barrier method (like a condom). The app takes a conservative approach to assessing fertility and classifies indecisive temperature measurements as red/fertile by default. For someone with a regular period, the app shows red days from day 6 to day 16 of the menstrual cycle, on average. Natural Cycles is not a “stand-alone” contraceptive method. It must be paired with either abstinence or other methods of non-hormonal contraception on red days.

It can take up to three months for the app to ‘learn’ your likely menstrual cycle. Natural Cycles’ maker claims the app is “effective from day one,” but that simply means that the app displays more red (use protection) days than green days until it has gathered enough data. On red days, you must either use a barrier method for protection or abstain from intercourse altogether.


Natural Cycles brand influencers on social media promote the app but don’t always make its limitations clear. Waking up in the middle of the night to care for young children can throw off your BBT.

Effectiveness & Limitations

Traditional fertility awareness methods have a “typical use” failure rate of 24%—meaning, out of every 100 people using the method, 24 will become pregnant in a year. In contrast, Natural Cycle’s developers claim that the app has a “typical use” failure rate of 6.5%, which they attribute to the accuracy of the app’s algorithm.

However, there’s reason to worry that the failure rate may be much higher. Natural Cycles was first released in Europe, where women have reported issues with the app’s usability and false sense of reliability. The FDA disregarded these reports in their expedited clearance of Natural Cycles but the National Women’s Health Network is concerned that the app’s true effectiveness could be far lower than advertised. And in fact, the FDA itself warns that “Natural Cycles should not be used by women who have a medical condition where pregnancy would be associated with a significant risk to the mother or the fetus.” A new vaginal ring, Annovera, approved on the same day as Natural Cycles with a similar typical use failure rate did not carry this same warning, suggesting that even the FDA may believe the app’s real failure rate is higher.

Furthermore, using a hormonal contraceptive—including emergency contraception, like Plan B—can throw off the app’s calculations. Natural Cycles is most effective for people who have not been on hormonal birth control within 12 months prior to using the app. If you have used hormonal contraception—such as the pill, patch, ring, shot, implant, or hormonal IUD—within 60 days of starting Natural Cycles, you are at increased risk of unintended pregnancy.

Likewise, if you have an irregular menstrual cycle for any reason—for example, polycystic ovarian syndrome (PCOS)—or have recently been pregnant or had endometriosis, Natural Cycles is not recommended for you.

Similarly, it is likely that Natural Cycles was only tested on cis women and the effects of hormone therapy on the app’s success rate are unknown. However, because androgens can affect both the menstrual cycle and basal body temperature, if you are undergoing transgender hormone therapy, talk to your doctor before using Natural Cycles.


Privacy Concerns

The Natural Cycles database stores highly personal medical information, including information that can identify you, as well as data on your sex life, pregnancy goals, contraceptive use, body temperature, menstrual cycle, dates of intercourse, ovulation test results, and pregnancy test results. The app it is not required to comply with medical information privacy laws. The app’s privacy policy permits its owners to sell user information to third parties and notes that it “may disclose your Personal Data in order to comply with a legal or regulatory obligation.” Learn more about the potential outcomes of data sharing or a privacy breach of Natural Cycles.


Partnership with Oura Ring for “Wearable Birth Control”

In November 2020, Natural Cycles announced a partnership with Oura Health, a separate company that makes a wearable device worn on the finger. Instead of using a traditional basal body temperature (BBT) thermometer placed under the tongue each morning for use with the Natural Cycles app, the ring would track users’ BBTs overnight and upload that data to the app. (Consumers currently have this option via a beta program.)

As noted above, the NWHN has been concerned about the privacy risks and true effectiveness of the app from its inception, and the integration of Oura into the Natural Cycles system only exacerbates our concerns. Potential users should be aware that the Oura ring has not been cleared by the FDA as a medical device and there’s little independent data about the Oura ring’s effectiveness. The studies that have been done thus far (like this one) were controlled by the company.

Because BBT can be impacted by many factors—including sleep disruptions and alcohol consumption—it’s not always an accurate fertility metric. On top of that, measuring BBT requires a greater degree of sensitivity than a fever thermometer. If the Oura ring isn’t accurate enough to track these subtle temperature changes while filtering out the temperature changes that are unrelated to fertility, the likelihood of false Natural Cycles predictions increases, ultimately increasing the chance of an unplanned pregnancy. Natural Cycles is currently only permitting fertility tracking users, not contraceptive users, to use the Oura ring, but their announcement promises that “Wearable Birth Control” is “coming soon.” Birth control users should know that they could be compounding the errors associated with the data used to predict when it’s safe to have sex without a backup form of contraception.

Finally, pairing the Oura ring with the Natural Cycles app ensures that two companies have access to some of the most private user data available, with few real constraints on how they use that data. If the ring is as accurate as its makers hope, it will mean both companies will know when a user is pregnant, or has had a miscarriage or an abortion.


The National Women’s Health Network strongly endorses the kind of body knowledge and self-care which Natural Cycles and other fertility awareness apps can foster. However, before switching to a contraceptive app, potential users should be fully informed about the potential limitations and trade-offs. The app’s ability to prevent pregnancy may be lower than advertised and the demands of accurately measuring BBT may make it inconvenient or unrealistic for many people.


Updated November 2020