The NFP rules of the symptothermal method compared to Daysy

The actual NFP rules, with the recording and assessment of certain body signs, were established in the early 1980s and then confirmed in studies by the German NFP working group "Natural Fertility." It was intended that users would be taught and internalize the NFP rules in certified courses. This is because when these rules were developed, digital training and algorithms did not yet exist. It took another six years before the first fertility tracker, Lady-Comp, was developed in 1986.

Given their common genesis, it is not surprising that NFP and the Daysy fertility tracker have parallels. But of course, traditional NFP rules have been modified by digital capabilities and advancements, in favour of ease of use and accuracy. The differences between traditional NFP rules and the symptothermal method rules of the Daysy smart NFP device are presented below.

Symptothermal method rules: Temperature measurement as a basis for determination

  • Traditional NFP rules: Following the symptothermal method rules. a woman's basal body temperature (BBT) is measured every morning with a special basal thermometer and the cervical mucus is examined. All results and other body signs are recorded in a standardized cycle curve1.
  • Daysy: Daysy also uses standardized evaluation models and curves, just like in the traditional NFP rules. However, this fertility tracker also uses a standardized thermal sensor. This enables very high measurement accuracy, precisely matched to the algorithm. Measurement or transmission errors as with the usual basal thermometers are thus excluded2. The thermosensor waits during each measurement until the final temperature has stabilized (see figure). If the body temperature drops shortly after a peak value (for example, because you take a deep breath in your mouth and inhale cold air during the measurement), this will not falsify the result - the temperature is not considered recorded until the value has stabilized. This assures the Daysy user of the highest possible measurement accuracy.

Symptothermal method: Determination of infertile and fertile days after menstruation

  • Traditional NFP rules: The so-called 5-day NFP rule specifies that the first five days in the first cycle can be assumed to be infertile. If the first higher temperature measurement occurs on the twelfth day of the cycle or earlier, the minus-8 rule of the symptothermal method rules applies immediately: After twelve documented cycles, the last infertile day at the beginning of the cycle is the day of the earliest first higher temperature measurement minus eight days.
  • Daysy: All symptothermal method rules rely on reliable data. Since Daysy does not have individual data available, especially at the beginning, it always assumes in the first cycle that also only the first 5 days can be assumed as infertile and that all other days until ovulation could be fertile. Daysy will adjust this status more and more individually over time and with more available data from the user, calculating infertile (green) days during the follicular phase by "learning" from previously entered data (menstruation) and daily basal body temperature. The earliest recorded day of ovulation, calculated by the corresponding characteristic increased basal body temperature, is the starting point for the days after menstruation indicated as infertile (green).

The determination of the infertile days after ovulation

  • Traditional NFP rules: In the symptothermal method, the BBT is said to have increased if, on three consecutive days, readings are obtained that are all higher than those of the previous six days. The third higher value must be at least 0.2°C (0.9°F) above the highest value of the previous six days (3-over-6 rule). In practice, there are also two exceptions to the BBT rise NFP rule or to the identification of the day on which BBT first rose:
  1. If, in the case of increased BBT, the third value is less than 0.2°C (0.9°F) above the highest value of the previous six days, a fourth day must be waited on. On this day, the value must also be higher than the six days preceding the BBT increase, but not necessarily 0.2°C (0.9°F) higher.
  2. Between a total of three elevated BBT readings, there may be one value that drops so far that it is at the level of the days before the BBT rise (or even lower). Then this lower value can be ignored. However, the total third elevated value must be at least 0.2°C (0.9°F) above the average of the six days before the BBT rise.
  • Daysy: Following the symptothermal method rules, the fertility status after ovulation is also determined via BBT. However, constant comparison with statistically determined values is used to calculate when the transition from infertile to fertile days occurred. In addition, the BBT measurement results are used: if a significant increase of at least 0.2 °C (0.9°F) was detected at the expected time of ovulation, or if the BBT was above the previous phase for at least two consecutive calendar days, it can be assumed that ovulation has occurred. Daysy assigns an infertile status to the following days. However, not only three days, but each calendar day is evaluated, and it is always compared with the previous values to determine how likely it is that the user is in the postovulatory luteal phase (during which BBT remains elevated). Daysy now waits until the user enters that her menstruation has started - or until it turns out that the phase with elevated basal body temperature lasts longer, which may indicate pregnancy.

Of course, there are always cycle shifts, or a user forgets to enter the start of her period. In case of such irregularities, Daysy detects this as "status unknown" and displays a yellow light. This means that whether you are currently fertile or infertile is temporarily unclear. The yellow light will also be illuminated if the recorded data on a day cannot be assigned to any previously recorded scenario (e.g. during a febrile infection). Using the symptothermal method with Daysy, yellow days should always be treated like red days, and the user should therefore always assume possible fertility on these days.

Cervical mucus

The term "symptothermal" indicates this: The NFP method always records a second marker that complements basal body temperature. Either the change in consistency of the cervical mucus is determined or the cervix itself is palpated to check the position and firmness of the cervix.

  • Traditional NFP rules: Shortly before ovulation, the cervical mucus (i.e., the mucus at the cervix), which is otherwise rather firm, yellowish, and sticky, becomes clear and spinnable. That is, you can pull it apart between your fingers and watch it pull threads. Optimal cervical mucus - clear, thin and spinnable - indicates the fertile days. As soon as the consistency of the cervical mucus decreases again, it becomes darker and thicker, the postovulatory phase can be assumed. To know exactly when the fertile days have occurred, it takes one to several days of optimal cervical mucus quality, followed by three days of poorer quality.
  • Daysy: Not only in the symptothermal method, cervical mucus is a very important fertility marker for women who have experience monitoring their body signs. The condition of cervical mucus can be recorded in the DaysyDay app - this is an extra offer to anyone who wants to include multiple body signs in their family planning. However, because cervical mucus looks different for every woman and because our international surveys have found that even participants in NFP training are often unsure how to rate their cervical mucus quality - this information is not included in the evaluation algorithm. The Daysy device was designed for ease of use independent of training, eliminating potential human error as much as possible.

From a scientific point of view, there is currently no standardized, digital method to objectively determine mucus quality - especially since various factors can influence the composition of mucus. Medications, dehydration (alcohol), or coffee consumption, among others, should be mentioned. All this makes interpretation complex and cervical mucus alone less suitable for everyday use as a fertility marker when using the traditional symptothermal method.


Another body sign that can give women clues about their fertility status: changes in the cervix, or more precisely, the external cervix. During infertile days, it is hard, and its position slightly lower; thus, it is easy to feel on these days. Fertile days are characterized by a softer and higher position of the outer cervix, which opens towards ovulation. At this time, the cervical mucus also flows.

  • Traditional NFP rules: During the cycle, regular palpation usually reveals how the cervix changes in position and firmness, and when the cervix opens.
  • Daysy: The cervix is as individual as cervical mucus, and cervical position and changes vary from woman to woman. There are no universal standards to describe position and firmness. In addition, the cervix and cervical mucus change due to childbirth and menopause.

Other differences

The Daysy symptothermal method device not only focuses on the current cycle, but also includes past cycles in its calculations. This makes it easier to predict fertile days for couples trying to conceive before ovulation occurs.

A recent study evaluating Daysy's miscalculated infertile days concluded that only 0.6% of the days indicated as infertile were false negatives and were within the fertile window.

Daysy is an intelligent fertility tracker that lets you get to know your very own menstrual cycle.


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.