Natural Cycles is a smartphone app that uses your daily body temperature to determine which days you can and can’t get pregnant. It’s the first app to receive FDA clearance as a contraceptive device, classified as a Class II medical device, the same regulatory tier as condoms and diaphragms. Instead of hormones or physical barriers, it works by pinpointing your fertile window each cycle and telling you when to use protection or abstain.
How the Algorithm Works
Every morning, you take your temperature before getting out of bed and log it in the app. After ovulation, your body releases progesterone, which causes a small but measurable rise in your resting (basal) body temperature. Natural Cycles detects that shift to confirm ovulation has occurred. Over time, the algorithm learns your individual ovulation pattern and uses it to predict when ovulation will happen in future cycles.
The app calculates your fertile window by working backward from the predicted ovulation day. Since an egg survives roughly 24 hours after release and sperm can live up to five days in the reproductive tract, the fertile window spans about six days. The algorithm then adds a safety margin on either side, sized according to how regular or irregular your personal cycle tends to be. You can also log the results of a urine-based hormone test to give the algorithm more data, but this isn’t required.
Red Days and Green Days
The app boils everything down to a simple daily status. Green days mean you’re not fertile and can have unprotected sex. Red days mean you’re likely fertile: you either need to use a barrier method like condoms or avoid sex entirely to prevent pregnancy. The app is clear that predictions for upcoming days can shift as new temperature data comes in, so you should check your status each morning rather than relying on a forecast from earlier in the week.
Early on, when the algorithm is still learning your cycle, expect more red days. As it gathers several months of data and identifies your pattern, green days typically increase.
How Effective It Is
The traditional basal body temperature method, which Natural Cycles is built on, is up to 99% effective with perfect use. “Perfect use” means measuring every morning, logging accurately, and consistently using protection or abstaining on every red day. The gap between perfect use and typical use matters here more than with most contraceptives, because the method depends heavily on daily compliance. Missing a morning measurement, logging after a night of poor sleep, or skipping protection on a red day all erode effectiveness. For context, typical-use failure rates for fertility awareness methods as a category generally fall in the range of 2 to 12 pregnancies per 100 women per year, depending on the method and how consistently it’s followed.
Who It Works Best For
Natural Cycles is most reliable for people with reasonably regular menstrual cycles who have a consistent daily routine. The temperature reading needs to be taken at roughly the same time each morning, before physical activity, and after a stretch of uninterrupted sleep. That makes the app a poor fit for certain lifestyles and conditions.
Rotating shift work is one significant factor. Research shows that women with 20 or more months of rotating shift work are about 23% more likely to have irregular cycles and roughly 49% more likely to have very long cycles (40+ days), with a dose-response pattern: more months of shift work, more disruption. Irregular cycles make ovulation harder to predict, which widens the margin of error for any temperature-based method. Similarly, conditions that cause irregular ovulation, frequent temperature fluctuations from illness or alcohol, or hormonal contraceptive use that hasn’t fully cleared the system can all reduce the algorithm’s accuracy during the learning phase.
Taking Your Temperature
You can measure with a two-decimal-place basal thermometer (the kind designed for fertility tracking, not a standard fever thermometer) or skip the manual step entirely by using a wearable. Natural Cycles integrates with the Oura Ring (Gen 2, Gen 3, and Ring 4), which measures your skin temperature overnight using sensors that capture 1,440 data points per day, precise enough to detect changes as small as 0.13°C (about 0.23°F). The ring syncs temperature trend data to the Natural Cycles app each morning after you open the Oura app. Period data and other Oura metrics are not shared between the apps, so you still log your period manually in Natural Cycles.
The wearable option removes the biggest friction point. There’s no alarm to remember, no thermometer to reach for before sitting up, and no risk of forgetting on groggy mornings. It does require active subscriptions to both Natural Cycles and Oura.
Cost and Payment
Natural Cycles offers two subscription tiers: a monthly plan and a discounted annual plan. For users in the United States, both plans are eligible for reimbursement through an FSA or HSA. After signing up, you can request an itemized receipt at checkout and submit it to your benefits provider. The company also partners with FSAstore.com, where you can purchase a subscription directly with an FSA or HSA card.
Data Privacy
Because the app collects sensitive reproductive data, its privacy protections are worth understanding. Natural Cycles encrypts your information in transit and at rest, and uses pseudonymization, replacing your name and email with codes so that your fertility data isn’t directly tied to your identity in their systems. You can enable Face ID on iOS for an extra login barrier.
The app also offers a feature called Go Anonymous, which fully separates your identity from your fertility data so that even Natural Cycles’ own team cannot link the two. You can request deletion of your data at any time, and the company says it anonymizes personal data when there’s no longer a reason to retain it. Any data used for research purposes is aggregated and anonymized, and requires your explicit consent. As a regulated medical device, the app is designed to comply with GDPR standards and undergoes regular security audits.
Using It to Plan a Pregnancy
Natural Cycles also has a “Plan a Pregnancy” mode that flips the logic. Instead of flagging fertile days as days to avoid sex, it highlights them as your best opportunity to conceive. The underlying algorithm is the same: it identifies your fertile window based on your temperature pattern and optional hormone test results. For people already comfortable with the app’s tracking routine, switching modes is straightforward and gives you cycle-specific timing guidance without needing a separate fertility tracking tool.