The Fertility Tracker Method Precise Cycle Tracking With Intelligent Technology

The Fertility Tracker uses a highly precise sensor designed specifically for basal body temperature (BBT) measurement. Unlike conventional thermometers, this sensor does not provide an instant reading. Instead, it waits for the final temperature value to stabilize, ensuring maximum accuracy. This is why the Fertility Tracker typically requires about 60 seconds to complete a measurement.

Your basal body temperature does not change abruptly; rather, the sensor must gradually warm up until it reaches the final temperature value. During this warming phase, the temperature reading follows a rising curve toward stabilization. Many other BBT monitoring devices use rapid measurement techniques by extrapolating this curve, essentially “guessing” or approximating the final value.

The Fertility Tracker, however, prioritizes accuracy over speed. It waits until the final stable value is reached, ensuring a reliable reading. If the temperature drops momentarily—such as when cold air enters the mouth during measurement—the device does not record the value until it rises again and stabilizes.

This method guarantees the most precise temperature measurement possible, forming the foundation for accurate fertility calculations and the overall reliability of the Fertility Tracker Method.

The algorithm of a fertility tracker begins each new cycle on the first day of menstruation. From this point forward, daily Basal Body Temperature measurements are taken to monitor the course of the cycle, particularly the pre-ovulation phase. Once a characteristic temperature rise is detected, ovulation is considered likely.

During the first few cycles, when there is insufficient individual data, the algorithm assumes that all days following menstruation until ovulation may be fertile. As a result, the device initially displays more red days—days when conception is possible (see Figure A).

With continued use, the algorithm becomes increasingly personalized. Based on daily BBT readings and previously entered cycle data, the algorithm gradually calculates infertile days before ovulation. During this learning phase, the fertility tracker collects all relevant data to precisely identify ovulation. This leads to a fertile window that typically begins five days before the predicted ovulation.

The algorithm continuously compares current temperature values with both historical cycle data and a comprehensive cycle database built from over 40 years of market presence—a unique resource unmatched by any other provider worldwide. Using this extensive data and advanced analysis, it determines the earliest possible day of ovulation. The fertile phase only ends once a distinct temperature shift has confirmed ovulation.

After the temperature rise has been confirmed, the algorithm marks all subsequent days as infertile. Each day, the system re-evaluates whether the user remains in the luteal phase. A new cycle begins once menstruation is recorded—or a prolonged high-temperature phase may indicate a possible pregnancy (see Figure B).

The underlying algorithm has been continuously optimized for over 40 years. It is based on the analysis of more than 50 million evaluated cycles and is used in all Valley Electronics devices, including Lady-Comp, Baby-Comp, Pearly, Daysy and Teena.

The classic Temperature-Only Method is focused on the phase after ovulation, assuming that all preceding days are potentially fertile. This significantly reduces the number of days considered reliably infertile. In contrast, the Fertility Tracker Method uses both current and historical data, allowing it to distinguish between fertile and infertile days even in the first half of the cycle with greater accuracy.

The result: Instead of only 20–30% infertile days (including menstruation), as with the classic method, the Fertility Tracker Method achieves an almost balanced 50:50 distribution of fertile and infertile days in women with regular cycles.

The calculation of the Calendar Method for the fertile window is based on the average length of previous cycles. It assumes that the post-ovulation phase—the second half of the cycle—always lasts 14 days (1). The fertile window is then estimated to open four days before and close three days after the predicted ovulation date.

However, this method does not take into account real-time information from the current cycle (such as daily basal body temperature measurements). Since ovulation—and therefore the individual fertile window—can vary by an average of 5 days within a year, this method is considered inaccurate (2).

The classic Calculothermal Method combines basal body temperature (BBT) measurements with Ogino’s calendar calculation. The start of the fertile phase is determined by subtracting 18 days from the shortest cycle length, while the start of the infertile phase is identified using temperature data (3).

While the classic Calculothermal Method relies on a rigid structure, the Fertility Tracker Method leverages recorded cycle data to determine fertile days after menstruation with greater precision. The Calculothermal Method attempts to predict the fertile window, whereas the Fertility Tracker Method—due to the initial lack of sufficient data—assumes that all days following menstruation could be fertile during the first cycles. These fertile days are then adjusted and refined individually with each cycle.

Once ovulation is presumed to have occurred, the Fertility Tracker Method uses measured BBT data to assign a mathematical, post-ovulatory infertile status to the following days. This mathematical evaluation is reassessed daily to determine whether the user remains in the infertile luteal phase.

(Based on scientific research in Natural Family Planning (NFP), Valley Electronics has developed a unique method that enables a new type of evaluation through statistical and mathematical analysis.)

Wearables like the Apple Watch 8, Oura Ring, or Fitbit measure peripheral skin temperature, which is prone to environmental and physiological fluctuations, making it unreliable for precise fertility tracking. A pilot study revealed that although a general correlation to cycle phases was observed, the temperature rise at ovulation showed a considerable range of variation. In 18% of 437 confirmed ovulatory cycles, no temperature rise was detected at all, and in another 5%, ovulation occurred outside the predicted fertile window, leading to potential safety-related misdiagnoses. Another study showed ovulation fell within the predicted fertile window in only 83% of cycles, which is insufficient for accurate fertility determination.

In contrast, Daysy and other fertility trackers provide over 99% accuracy in distinguishing fertile from non-fertile days. This high accuracy is based on oral basal body temperature measurements taken in the morning, which remain consistent across all women and are not influenced by factors such as body fat percentage, ambient temperature, or seasonal changes—all of which can affect skin temperature.

While advancements in peripheral BBT tracking have improved its ability to roughly determine the cycle phase, it remains unsuitable for accurately distinguishing fertile from infertile days, a task where fertility trackers excel.

The Fertility Tracker Method does not include cervical mucus in its calculations for several important reasons:

• Subjectivity of Observation – Cervical mucus tracking is highly subjective, as it depends on the individual’s ability to correctly recognize and interpret changes in mucus texture and consistency. Studies show that misinterpretation of cervical mucus signs is common, leading to incorrect assessments of fertile days. Research highlights that the cervical mucus method often has a high learning curve and error rate when used alone.

• Cervical Mucus Is Not Always Present – Cervical mucus production can fluctuate and is not consistent throughout all fertile years. Hormonal changes, stress, infections, or medications can impact the presence and quality of cervical mucus, making it an unreliable indicator for some women.

• Difficult Entry for Beginners – Many women find cervical mucus analysis uncomfortable, confusing, or simply difficult to integrate into their daily routine. This complexity often leads to improper use or discouragement from continuing with NFP methods.
• Algorithm Accounts for Buffer Days – The Fertility Tracker Method does not require cervical mucus observations, as its algorithm includes buffer days at the beginning of the cycle to account for potential early ovulation. This built-in safety margin ensures that fertility status is accurately determined without the need for mucus tracking.