07/03/2025 2:36 p.m.

The NFP rules of the symptothermal method compared to fertility tracker Daysy and Lady-Comp

Dr. Niels van de Roemer
Dr. Niels van de Roemer Medical Adviser

Classical NFP Rules vs. Fertiliy Trackers

Natural Family Planning (NFP) and the symptothermal method have been widely used for decades to help women understand their fertility cycles. The symptothermal approach relies on the observation of multiple fertility indicators, such as basal body temperature (BBT) and cervical mucus, to determine fertile and infertile phases. With the advancement of digital technology, fertility tracking devices like Lady-Comp and Daysy have revolutionized the process, offering automated and highly accurate cycle analysis. Below, we examine in detail the NFP rules of the traditional symptothermal method and how they compare to the advanced, algorithm-based evaluation system used by Daysy and Lady-Comp.

Evolution of NFP and the Birth of Digital Fertility Trackers

The modern NFP rules were formalized in the early 1980s by the German working group “Natural Fertility,” which standardized the recording and assessment of specific fertility signs. At the time, fertility tracking was taught through structured courses, as there were no digital tools to assist women in interpreting their cycle data. The launch of the first digital fertility tracker, Lady-Comp, in 1986, marked a turning point, paving the way for advanced devices like Daysy, which combine science-based algorithms with high-precision technology.

Given their shared origins, it is not surprising that Daysy incorporates the fundamental principles of the symptothermal method. However, digitalization has streamlined the process, minimized user errors, and increased the accuracy of predictions.

Temperature Measurement as the Basis

Traditional NFP Rules

  • Daily measurement of basal body temperature with a thermometer after waking up
  • Cervical mucus is examined
  • Values are documented on standardized cycle charts
  • Interpretation of the temperature curve is done manually

Daysy & Lady-Comp

  • Automated measurement with a high-precision thermal sensor
  • Measurement or transmission errors can therefore be excluded
  • The sensor waits for a stable temperature, preventing measurement errors caused by breathing or ambient temperature
  • Evaluation is carried out by an algorithm based on over 30 years of development

Advantage: Daysy & Lady-Comp eliminate transmission errors and provide maximum measurement accuracy without any paperwork.

Determining Fertile and Non Fertile Days After Menstruation

Traditional NFP Rules

The 5-day NFP rule assumes that the first five days of a woman’s cycle are infertile, provided that the cycle is at least 12 days long. After completing 12 documented cycles, the minus-8 rule comes into play: the last infertile day is determined by subtracting eight days from the earliest day of temperature rise observed in previous cycles.

Daysy & Lady-Comp

  • Also starts with the 5-day rule in the first cycle
  • After that, Daysy & Lady-Comp learn individually: Based on temperature data and menstruation entries, a personalized fertility pattern is calculated for each woman
  • Once Daysy & Lady-Comp has collected enough data, it can identify additional non-fertile days before ovulation.

Advantage: Daysy and Lady-Comp dynamically adapt to your cycle – the longer you use them, the more precise the predictions become.

Determining Infertile Days After Ovulation

Traditional NFP Rules

In NFP, ovulation is confirmed when there are three consecutive BBT readings higher than the previous six days, with the third reading being at least 0.9°F (0.2°C) higher than the highest value of the preceding six days (the “3-over-6 rule”). If the third day does not meet the 0.9°F requirement, a fourth day is observed for confirmation.

Daysy & Lady-Comp

  • Daysy & Lady-Comp also detect ovulation based on a temperature rise of at least 0.9°F (0.2°C)
  • They compare the current measurements daily with previously recorded cycle patterns to accurately determine when ovulation has occurred
  • Not just three days are evaluated, but every single calendar day, to better identify individual variations.
  • Once ovulation is confirmed, the display begins to show non-fertile days.
  • If the high-temperature phase persists unusually long, this may be an indication of pregnancy.
  • When unclear measurement data occurs (e.g., due to fever or incomplete entries), they display a yellow light – meaning: status unknown, potentially fertile.

Advantage: Daysy & Lady-Comp combine established rule-based knowledge with adaptive technology, allowing irregular cycles to be evaluated more accurately than with manual charting.

Cervical Mucus as a Fertility Marker

Traditional NFP Rules

The term symptothermal signifies the combination of BBT and another fertility sign—most commonly cervical mucus. Prior to ovulation, cervical mucus becomes clear, stretchy, and egg-white-like, signaling peak fertility. Post-ovulation, the mucus thickens and loses its elasticity, indicating the start of the infertile phase.

Daysy & Lady-Comp

Daysy allows users to track cervical mucus through its DaysyDay app, but this data is not included in the core algorithm. This decision was based on global research showing that many women, even after NFP training, find it difficult to consistently evaluate mucus quality. By focusing on temperature-based analysis, Daysy reduces subjective errors while still allowing users to record additional signs for personal awareness.

Cervix Position

Traditional NFP Rules

Another key sign in the symptothermal method is the position and firmness of the cervix. During fertile days, the cervix becomes softer, higher, and slightly open, while on infertile days it is lower and firmer.

Daysy & Lady-Comp

Does not require cervix palpation because this sign is highly variable between women and can be influenced by factors such as childbirth or hormonal changes. By relying on objective temperature data, Daysy and Lady-Comp ensure accuracy without the need for manual cervix checks.

Additional Benefits Of Daysy & Lady-Comp

Daysy and Lady-Comp combine the proven symptothermal method with advanced digital technology, offering a modern and highly reliable alternative to traditional NFP. Unlike manual charting, which requires daily interpretation of temperature and cervical mucus, these devices automate the entire process through high-precision thermal sensors and intelligent algorithms. By learning from both current and past cycles, they provide personalized fertility predictions, making them particularly valuable for women with irregular cycles or couples trying to conceive.

One of Daysy’s and Lady-Comp's greatest advantages is its ability to analyze historical data alongside real-time measurements, enabling accurate identification of fertile and non-fertile days. Scientific studies show that only 0.6% of Daysy’s infertile-day predictions fell within the fertile window, demonstrating its exceptional accuracy and reliability. Furthermore, the devices adapt dynamically to each user, refining predictions with every measurement, and even indicating potential pregnancy if the high-temperature phase persists longer than usual.

FAQs

What is the main difference between NFP and Fertility Trackers?

Traditional Natural Family Planning (NFP) methods rely on manual observation and recording of fertility signs such as basal body temperature and cervical mucus. Interpretation is done by the user using paper charts and set rules like the 3-over-6 rule. In contrast, Daysy and Lady-Comp automate the process with high-precision sensors and intelligent algorithms, providing immediate fertility status without the need for manual charting or interpretation.

Can Daysy and Lady-Comp be used by women with irregular cycles?

Yes. Daysy and Lady-Comp are designed to adapt to each individual’s cycle, even if it is irregular. Over time, these devices learn from both past and current cycle data to identify patterns unique to the user. This dynamic adjustment makes them more accurate and effective than manual NFP methods, especially for women whose cycles vary in length.

How accurate are Daysy and Lady-Comp?

A scientific study revealed that only 0.6% of the days marked as "infertile" by Daysy actually fell within the fertile window. This demonstrates an extremely high level of reliability. The combination of historical data analysis, daily measurement comparison, and algorithm-based evaluation significantly reduces the chances of false infertile-day predictions.

1.    Frank-Herrmann, P., Gnoth, C., Baur, S., Strowitzki, T. & Freundl, G. Determination of the fertile window: reproductive competence of women--European cycle databases. Gynecol. Endocrinol. Off. J. Int. Soc. Gynecol. Endocrinol. 20, 305–312 (2005).

2.    Händel, P. & Wahlström, J. Digital contraceptives based on basal body temperature measurements. Biomed. Signal Process. Control 52, 141–151 (2019).

3.    Roemer, N. van de, Haile, L. & Koch, M. C. The performance of a fertility tracking device. Eur. J. Contracept. Reprod. Health Care 0, 1–15 (2021).

4.    Lupinska-Dubicka, A. & Druz, M. J. A Comparison of Popular Fertility Awareness Methods to a DBN Model of the Woman’s Monthly Cycle. 8.

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