How Ava Works

How the Ava bracelet detects the fertile window

The Ava Fertility Tracker is for women who want to monitor their health for trying to conceive or getting to know their bodies.

The Ava bracelet is a completely unprecedented method of tracking a woman’s cycle. While you’re sleeping, Ava’s sensors collects data on key physiological parameters. Ava’s algorithm then detects your fertile window, physiological stress level, sleep quality, and more.

Other methods of fertility tracking rely on only a single parameter, and thus cannot provide a complete picture of your fertility in real time. Ava detects the very first signs that the fertile window is beginning, and confirms when it ends.

And because Ava is a wearable device, it provides accurate information about your cycle with minimal user effort. Wear Ava when you sleep, and wake up to insight.

  • Ava has been shown to detect an average of 5.3 fertile days per cycle at 89% accuracy (algorithm performance may be improved with more regular cycles, and diminished with less regular cycles).
  • Ava was clinically tested in a year-long study at the University Hospital of Zurich. Our first findings were published in the peer-reviewed journal Scientific Reports.
  • Ava is an FDA registered device and CE certified

A new way to track

Ava is worn every night while sleeping when the body is completely at rest and your physiological parameters are not influenced by activity

Sync the Ava bracelet with the app every morning.

See insight about your menstrual phase, sleep, stress and resting pulse rate. Continue tracking this data during pregnancy, along with weight tracking and weekly pregnancy content.

What Ava can measure

Skin temperature

The temperature of your skin at your wrist.

Skin temperature is one of the parameters Ava uses to identify your fertile window. It rises by about one half of a degree after ovulation, and remains elevated until your next period begins. During pregnancy, temperature remains elevated. If you’ve ever tracked your basal body temperature orally, you’ll notice that Ava’s temperature reading is several degrees lower than your oral temperature. It’s normal for skin temperature to be lower than oral temperature, but we found that it exhibits the typical biphasic pattern during the cycle.

Average skin temperature readings (when taken at a room temperature of 64.5 – 77 degrees Fahrenheit) are between 90.5 – 98.2 degrees Fahrenheit.

For Celsius:

Average skin temperature readings (when taken at a room temperature of 18 – 25 degrees Celsius) are between 32.5 – 36.8 degrees Celsius.

Resting pulse rate

How often your heart beats per minute at rest.

Scientific literature shows that resting pulse rate is lower during the follicular phase than the luteal phase. Ava’s clinical research partner, the University Hospital of Zurich, identified further variation in resting pulse rate throughout the menstrual cycle: resting pulse rate rose by about 2.1 beats per minute at the beginning of the fertile window.

During pregnancy, resting pulse rate remains elevated.

Typical resting pulse rates are between 40 and 80 bpm. If you are very physically active, your resting pulse rate may be even lower than 40 bpm. Stress, illness, and alcohol can cause temporary increases in resting pulse rate.

Breathing rate

Rate of respiration per minute.

Scientific literature shows that breathing rate is higher in the luteal phase than in the follicular phase. Breathing rate can vary widely during the course of a day due to your activities, but typical breathing rates during sleep are much more stable, and range from 10 – 25 bpm.

Heart rate variability ratio

The lower your HRV ratio, the less stressed you are.

HRV is the variation in the time interval from one heartbeat to the next. It can be used as an indicator of physiological stress: when there is a lot of variation in the time interval between heartbeats—high HRV—it means you are more resilient and less stressed. Ava measures the ratio between low frequency and high frequency waves in your heart rate. Because Ava measures HRV using this ratio, a lower number means you are less stressed.

Normal HRV ratio varies widely from person to person, but by looking for an increase or decrease from your baseline HRV ratio, you can learn about your body’s physiological stress level.


The process of supplying blood to the tissues of your body.

As blood flows through your capillaries, it delivers nutrients to the tissues and helps sweep away waste. Then, the blood flows back to the heart, and begins the process all over again.

If you’ve ever had an elastic band around your finger and felt the blood flow being cut off, you’re already familiar with the process of perfusion. While you can limit perfusion with an elastic band, perfusion can also change naturally for different reasons. For example, your perfusion changes in order to keep you warm or cool you down. Perfusion also changes in relation to the menstrual cycle: it is higher in the fertile days and lower during the luteal phase.


Ava tracks your movement with an accelerometer.

This parameter allows Ava to distinguish between light and deep + REM sleep.


Total sleep hours and ratio of light to deep + REM sleep.

The quality and quantity of sleep you’re getting each night are important parameters to track for your fertility. Your menstrual cycle can affect your sleep, and your sleep, in turn, can affect your menstrual cycle. Pregnancy can also influence sleep.

Ava records the amount of sleep you get each night. Additionally, Ava tracks the percentage of combined deep and REM sleep. Deep sleep is important for feeling rested the next morning, while REM sleep is important for learning and storing memories.

Typical sleep cycles for adults include 50 – 65 percent light sleep and 35 – 50 percent deep sleep + REM.

Data Quality

Ava must be worn for at least four hours per night with three hours of sleep in order to work properly.

Ava measures the physiological changes that happen when your body is completely at rest. It takes at least four hours for these parameters to stabilize and for Ava to get an accurate reading. If Ava recorded less than four hours, it won’t use the data from that night in calculating your fertility.

If you wore Ava for more than four hours but are still seeing “data quality poor,” it could mean the battery died during the night or that the sensors lost contact to your skin. Make sure Ava is fully charged when you put it on. This is indicated by a green light on the top of the bracelet.

The science behind Ava

Using physiological parameters to track a woman’s cycle and health

Ava is the first product that uses resting pulse rate, along with other physiological parameters, to detect the fertile window in real-time.

Most fertility monitoring products are based on a single parameter, providing an incomplete picture of the user’s fertility status. These parameters include basal body temperature and urinary LH. Used alone, none of these methods is able to accurately detect both the beginning of the fertile window and confirm ovulation. Furthermore, the reliability of a single parameter may be compromised by confounding factors. For these reasons, many women use multiple fertility tracking methods simultaneously, a tedious and time-consuming process.

Unlike other methods for tracking fertility, Ava detects changes that occur at the beginning of the fertile window as well as after ovulation, removing the need for the user to track multiple parameters herself while also providing more accurate predictions.

Clinical Study

Ava was tested in a year-long clinical study at the University Hospital of Zurich. Ava was found to detect an average of 5.3 fertile days per cycle, at 89 percent accuracy (algorithm performance may be improved with more regular cycles, and diminished with less regular cycles).

Results from the clinical study identified multiple physiological parameters that correlate with both the rise in estradiol that precedes ovulation as well as the rise in progesterone that follows ovulation. Ava has protected these methods with multiple patent applications. The first findings from the study were published in May 2017 in the peer-reviewed journal Scientific Reports. A significant increase in pulse rate was observed during the fertile window compared with the menstrual phase (2.1 beats per minute). Moreover, pulse rate during the mid-luteal phase was also significantly elevated compared with the fertile window (1.8 beats per minute).

The correlation between pulse rate and the fertile window

Earlier clinical studies demonstrated that there is a significant elevation in pulse rate measured in the clinic during the fertile window compared to the menstrual phase1. A prospective observational clinical trial was conducted to test whether wrist-worn photoplethysmography (PPG) sensors could reproduce this clinically observed association.

Indeed, the results from the clinical trial indicate that pulse rate measured with wrist-worn PPG sensors is significantly elevated during the fertile window compared to the pulse rate during the menstrual phase. The advantages of using wearable sensors are numerous, including the ability to continuously monitor the pulse rate during sleep thus minimizing the interference imposed by the various daily activity.

Our clinical trial also showed that the correlation between pulse rate and the different menstrual phases was robust to a broad range of activities the participants engaged in before sleep.

Most importantly, Ava has developed algorithms that allow us to detect the fertile window even for individuals with highly variable cycle length, a feature that is not attainable using the calendar method.

In conclusion, Ava offers science-backed fertile window detection that is suitable for a range of users with cycle lengths between 24 – 35 days in a user-friendly, accessible, and affordable package.

Frequently Asked Questions

  • I have irregular cycles. Can I still use Ava?

    As long as your cycles tend to fall within 24 – 35 days, you can still use Ava. Future versions of Ava will support more irregular cycles.

  • How accurate is Ava?

    Ava was shown in a year-long clinical study to recognize an average of 5.3 fertile days per cycle, at 89 percent accuracy.

  • Can Ava be used to prevent pregnancy?

    No, Ava cannot be used for contraception. Since our focus is helping women get pregnant, our algorithm identifies the most fertile days of the cycle. For a contraceptive product, we’d want to also identify days where it’s unlikely but still possible to get pregnant.

    That said, if you’re practicing NFP or FAM, Ava could be a great complementary tool.

Read more faqs


Get your Ava

Take charge of your cycle with Ava. No other cycle tracker detects five fertile days per cycle or provides insight about sleep, stress, and resting heart rate.

Order ava
By continuing to use the site, you agree to the use of cookies. more information Accept

This site is using first and third party cookies to be able to adapt the advertising based on your preferences. If you want to know more or modify your settings, click here. By continuing to use the site, you agree to the use of cookies.