on August 10, 2025
The Fertility Tracker Method (FTM) is a fertility awareness-based approach that has been automated within a single integrated device, combining a basal body temperature sensor with a self-learning algorithm. Rather than requiring the user to interpret data manually, the device does the calculation work, removing the risk of human error that affects other fertility awareness methods.
FTM draws on the logic of several established manual methods, including the Calendar Method, the Calculothermal Method, and the Symptothermal Method, and applies them through an automated process. The result is a more reliable and consistent assessment of fertile and infertile days.
The first Fertility Tracker was developed by Dr. Hubertus Rechberg in 1986. His company, Valley Electronics, continues to develop and refine fertility tracking technology worldwide, including Lady-Comp, Baby-Comp, and Daysy.
The device tracks and stores the user's basal body temperature each day, then independently calculates fertile and infertile days using that data. The self-learning algorithm continuously cross-checks its results against previously collected data, improving its ability to distinguish fertile from infertile days over time.
To use the Fertility Tracker Method accurately, all a user needs to do is measure their basal body temperature each morning and enter their menstruation days. The device handles everything else.
The Fertility Tracker Method is based in part on the Calculothermal Method and combines three core elements:
- Recording and learning from new data: daily basal temperature, menstruation start and end dates, and accumulated historical cycle data.
- Statistically significant evaluation of the post-ovulation temperature rise, using both current and historical data from the cycle database, via an algorithm calibrated to the female cycle.
- Elimination of human input and interpretation errors by integrating hardware (the sensor) and software (the algorithm) into a single device.
The Hardware: The Temperature Sensor
The Daysy and Lady-Comp fertility trackers use a highly precise sensor for basal body temperature measurement. Unlike most devices, it waits for the temperature reading to fully stabilise rather than extrapolating from a rising curve. This is why a measurement can take up to 60 seconds.
Many other BBT devices estimate the final temperature value from the initial rise. The Fertility Tracker waits for the value to stabilise completely. If the temperature dips during measurement, for example due to breathing or cold air entering the mouth, the reading is not recorded until it rises again and holds steady. This approach produces the most accurate temperature measurement possible.
The Software: The Self-Learning Algorithm
The algorithm uses the menstrual start date as the beginning of each new cycle. Daily BBT measurements establish the pre-ovulation phase until a characteristic temperature shift is detected. In the first few cycles of use, the algorithm has limited personal data to work with, so it treats all days after menstruation up to ovulation as potentially fertile. This is why more red days appear early on (figure A).
As the device accumulates data over time, it begins identifying infertile days in the pre-ovulation phase based on your personal temperature patterns and cycle history. Once enough data has been gathered, the tracker pinpoints your likely ovulation window and opens your personal fertile phase 5 days before your earliest predicted ovulation.
Once ovulation is identified through a sustained temperature shift, the algorithm assigns post-ovulatory infertile statuses to the days that follow. Each new day, a statistical re-evaluation determines whether you are still in the elevated-temperature luteal phase. The cycle closes when menstruation is entered, or when a prolonged high-temperature phase suggests possible conception (figure B).
How FTM Compares to Other Methods
The Temperature-Only Method
The Temperature-Only Method focuses exclusively on the post-ovulatory phase. The entire pre-ovulation phase is treated as potentially fertile. For women with relatively regular cycles, this means roughly 20 to 30% of days are identified as infertile (including menstruation). With the Fertility Tracker Method, that distribution is closer to fifty-fifty for women with regular cycles.
The Calendar Method
The Calendar Method calculates the fertile window based on average cycle length from previous cycles. It assumes the post-ovulation phase is always 14 days long and does not incorporate any data from the current cycle. Because ovulation can shift by an average of 5 days within a single year, this method offers limited accuracy.
The Calculothermal Method
The classic Calculothermal Method combines BBT with Ogino's calendar calculation, using the shortest cycle minus 18 days to identify the start of the fertile window, and temperature data to identify its end.
Valley Electronics built on this foundation to develop a more dynamic approach. Where the Calculothermal Method uses a fixed framework, the Fertility Tracker Method actively learns from each cycle, extending infertile days in the pre-ovulation phase as individual data accumulates. The Calculothermal Method predicts the fertile window; the Fertility Tracker Method refines it continuously based on real data.
A Note on Wearable Devices and Peripheral Temperature
Wrist-based and ring-based wearables such as the Apple Watch, Oura Ring, and Fitbit measure peripheral (skin) temperature rather than oral basal body temperature. Research has found that while some correlation to cycle phases exists, the temperature rise at ovulation shows considerable variation. In 18% of confirmed ovulatory cycles studied, no temperature rise was detected at all. In a further 5%, there was a safety-related misdiagnosis. A separate study found ovulation fell within the predicted fertile window in only 83% of cycles.
By contrast, Daysy achieves over 99% accuracy in distinguishing fertile from infertile days. Oral basal body temperature, measured consistently each morning, is a stable and reliable indicator across all women. Peripheral skin temperature can be influenced by body fat percentage, environment, and even season.
Wearable technology has made meaningful progress in cycle awareness, but it is not yet suitable for reliably distinguishing fertile from infertile days.
Daysy is an intelligent fertility tracker that lets you get to know your very own menstrual cycle.
A note from Period Wisdom Boutique
Period Wisdom Boutique exists because we believe women deserve to understand their bodies. Not in a clinical, overwhelming way, but in a way that feels personal, empowering, and genuinely useful in everyday life.
The Fertility Tracker Method is a good example of why the science behind a device matters. Knowing how Daysy or Lady-Comp actually works, what the algorithm is doing, why it takes time to learn your cycle, and why oral BBT outperforms wrist temperature, gives you confidence in the data and in your own body. That understanding is exactly what we are here to help you build.
