The Science Behind Ava

  • Ava has been shown to detect an average of 5.3 fertile days per cycle at 89% accuracy
  • Ava was clinically tested in a year-long study at the University Hospital of Zurich under the lead of Prof. Dr. Brigitte Leeners, a leading expert on the mathematical modeling of menstrual cycles.
  • Ava is an FDA Class 1 medical device.

Read a summary of our research.

What Ava Measures

Pulse rate is an accurate estimation of how often your heart beats per minute (bpm). Typical resting pulse rates for adults are between 40 and 80 bpm, but if you get a lot of aerobic exercise, your resting pulse rate may be lower.
Your wrist temperature is lower than your oral temperature, but just like your oral temperature, it changes based on your changing hormone levels. Though it may be hard for you to discern a pattern in your changing wrist temperature, Ava’s algorithm is trained to recognize these changes.

Average skin temperature reading (when taken at a room temperature of 64.5 – 77 degrees Fahrenheit) are between 90.5 – 98.24 degrees Fahrenheit. If you sleep in a room warmer than 77 degrees it could cause your skin temperature to go up.

HRV is the variation in the time interval from one heartbeat to the next. It can be used as an indicator of physiological stress.

Most people think of heart rate as the average number of beats per minute, but your heart rate changes from beat to beat. HRV measures this naturally occurring irregularity in heart rate. The more variation in the interval between beats, the lower your physiological stress level.

Ava measures the ratio between low frequency and high frequency waves in your heart rate. Scientific studies have shown that the HRV ratio is a good indicator to assess the balance between the sympathetic nervous system and the parasympathetic nervous system. An imbalance in these systems—usually when the sympathetic branch takes the upper hand—is associated with high physiological stress level and results in a high HRV ratio.

Estrogen and progesterone have an impact on HRV. The HRV ratio is increased during the luteal phase when progesterone peaks, compared with the follicular phase, when estrogen is high.

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. HRV ratio is complicated, but the important thing to remember that a higher HRV ratio indicates higher stress.

The quality and amount 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. Ava shows the total amount of sleep you get each night. This doesn’t count time you might spend reading in bed; the bracelet can distinguish between sleeping and lying quietly. It also tracks the percentage of light sleep and the percentage of combined deep and REM sleep.

Average percentages for adults are 47 – 60 percent light sleep and 33 – 48 percent deep sleep + REM.

Rate of respiration per minute.
Ava tracks your movement with an accelerometer. This parameter allows the bracelet distinguish between light and deep + REM sleep.
Perfusion describes 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. Estrogen and progesterone also have an influence on perfusion. Estrogen increases perfusion, while progesterone has the opposite effect.

Bioimpedance measures the resistance of body tissue to tiny electric voltages. If you’ve ever measured your body composition such as fat content, this is very similar. Besides analyzing fat content, bioimpedance provides information about the skin, including hydration and sweating patterns.

As you have probably observed throughout your life, reproductive hormones have an impact on your skin. Your skin can change at different phases of the menstrual cycle, during puberty, and during pregnancy.

Heat loss is closely related to the onset of sleep. As you lose heat through your hands and feet, your skin temperature increases and your core body temperature decreases. This heat loss is closely related to the onset of sleepiness.

Heat loss is also related to your metabolic rate. The mechanism for the maintenance of the higher temperature during the luteal phase is still a matter of scientific discussion. Some researchers believe that reduced heat loss is responsible, while others believe that higher internal heat production (increased metabolic rate) is the cause.

Our Clinical Study

Ava completed a year-long clinical study at the University Hospital of Zurich under the lead of Prof. Dr. Brigitte Leeners, a leading expert on the mathematical modeling of menstrual cycles. Ava was found to identify an average of 5.3 fertile days per cycle with an accuracy of 89 percent. Read a summary of our clinical study.

The clinical study used a wearable device to track nine physiological parameters throughout 155 menstrual cycles. The data gathered was cross referenced with urine tests taken during the fertile phase.

The results of the study were presented in June 2016 at the Swiss Society of Obstetrics and Gynecology Annual Congress and in October at the German Society of Obstetrics and Gynecology Annual Congress. Bayer, the leading women’s health company, is sponsoring Ava’s presentation.

Our scientific research paper will be submitted in summer 2016. We expect it to be published by a leading peer-reviewed journal in reproductive health by the end of 2016.

Ava is planning further clinical studies to refine its algorithms for use in both pregnancy recognition, pregnancy monitoring, and possible use as a non-hormonal contraceptive device.

How physiological parameters can be used to identify fertile days.

There are only a few days per cycle when it is possible for a women to conceive. These days vary from woman to woman and from cycle to cycle, but studies show that this time usually lasts six days, beginning five days prior to ovulation and lasting until the day of ovulation itself.

Not all women experience six fertile days. The fertile window is impacted by:

  • The lifespan of the egg, which is up to 24 hours after ovulation
  • The lifespan of the sperm. The median is 1.5 days, but when fertile-quality cervical mucus is present, some high-quality sperm can survive up to five days.

The fertile window begins with a gradual rise in estradiol levels over three days1, along with a moderate probability for conception. The two days that follow are characterized by high levels of estradiol in combination with a surge in luteinizing hormone. These two days, along with the day of ovulation, represent peak fertility, with a pregnancy probability of more than 20 percent. Seventy percent of pregnancies are conceived in these latter three days of the fertile window.

After ovulation, the likelihood of conceiving drops rapidly due to the limited lifespan of the follicle (typically 12-24 hours).2

Tracking physiological parameters associated with the rise in estradiol allows women to recognize the first days of the cycle when conception is possible, and provides advance warning of peak fertile days. Research shows that women who track their fertile days are twice as likely to conceive in a given month (compared with untimed intercourse once per week). 3

The rise in estradiol levels stimulates the secretion of fertile quality cervical mucus, which supports the survival and transport of sperm. Intercourse that occurs after estradiol levels rise may lead to conception, as some sperm can survive for several days in favorable environments.

Changes in reproductive hormone levels have measurable physiological impacts throughout the body. 4 For example, it has been understood for decades that progesterone has a direct impact on basal body temperature 5, causing a BBT spike by one half to one degree Fahrenheit after ovulation. Charting temperature with a BBT thermometer has been shown to be a moderately effective method for detecting when ovulation has occurred. 

Advancements in sensor technology make it possible to collect physiological data points continuously over long periods of time, enabling more precise monitoring of correlations between the hormones progesterone and estradiol with physiological parameters. The Ava fertility tracker is the first fertility tracking device to collect data continuously throughout the night, allowing it to detect an average of 5.3 fertile days per cycle with minimal user effort.

The Team

The Ava team brings together industry leaders in wearable technology, women’s health, and data science to deliver an unprecedented device for monitoring women’s health.

Prof. Dr. med. Brigitte Leeners
Prof. Dr. med. Brigitte LeenersScientific Advisory Board
After medical studies at the University of Aachen/Germany, Prof. Leeners became a senior physician at the University of Zurich/Switzerland. One of her main research topics is mathematical modelling of the menstrual cycle. Prof. Leeners’ research has been published in leading publications in the field of gynecology and reproduction medicine around the world. Prof. Leeners has been strongly involved in the scientific studies of Ava, and joined the scientific advisory board in 2015.
Lisa Falco, PhD
Lisa Falco, PhDVP Data Science
After an MSc in Engineering Physics from Chalmers University of Technology in Gothenburg/Sweden, Lisa did a PhD in biomedical signal processing at EPFL in Lausanne/Switzerland and was a visiting researcher at Harvard Medical School in Boston/US. Lisa worked in the algorithm and signal processing teams of Solianis (non-invasive measurement of blood sugar) and Scanco Medical (microcomputed tomography scanners). Lisa is the author of several patents and has presented scientific results at various international conferences.


  1. Fritz 2012
  2. Wilcox 1996
  3. Wilcox AJ, Weinberg CR, Baird DD. Timing of Sexual Intercourse in Relation to Ovulation. Obstet Gynecol Surv. 1996;51(6):357-358. doi:10.1097/00006254-199606000-00016.
  4. Hirshoren N, Tzoran I, Makrienko I, et al. Menstrual cycle effects on the neurohumoral and autonomic nervous systems regulating the cardiovascular system. J Clin Endocrinol Metab. 2002;87(4):1569-1575. doi:10.1210/jc.87.4.1569.
  5. Kräuchi K, Konieczka K, Roescheisen-Weich C, et al. Diurnal and menstrual cycles in body temperature are regulated differently: A 28-day ambulatory study in healthy women with thermal discomfort of cold extremities and controls. Chronobiol Int. 2014;31(1):102-113. doi:10.3109/07420528.2013.829482.