The NFP Rules Of The Symptothermal Method Compared To Daysy

Natural Family Planning (NFP) rules were formalised in the early 1980s, developed by the German NFP working group "Natural Fertility" and validated through clinical studies. At the time, they were designed to be taught in certified courses — because digital tools and algorithms simply didn't exist yet. It wasn't until 1986 that the first fertility computer, Lady-Comp, was introduced, bringing these principles into the digital age.

Given this shared foundation, it's no surprise that Daysy and Lady-Comp draw on the same core principles as traditional NFP. But digital technology has allowed those principles to evolve — improving both accuracy and ease of use. Here's how the two approaches compare.

Temperature Measurement: The Foundation of Both Methods

• Traditional NFP: Each morning, a woman measures her basal body temperature (BBT) using a specialist basal thermometer and observes her cervical mucus. All readings and body signs are recorded manually on a standardised cycle chart.
• Daysy: Daysy uses the same standardised evaluation models as traditional NFP, but replaces the manual thermometer with a precision thermal sensor calibrated specifically to its algorithm. This eliminates measurement and transcription errors common with standard basal thermometers. The sensor waits for the temperature to fully stabilise before recording — so even if you briefly inhale cold air during measurement, the result isn't affected. The outcome is the highest possible measurement accuracy, every time.

Identifying Infertile Days After Menstruation

• Traditional NFP: The classic "5-day rule" treats the first five days of the cycle as infertile. After twelve documented cycles, the "minus-8 rule" applies: the last infertile day at the start of the cycle is calculated as the earliest recorded first higher temperature day, minus eight days.
• Daysy: Like traditional NFP, Daysy starts conservatively — treating only the first five days as infertile in a new user's first cycle. Over time, as more data is collected, Daysy's algorithm learns from each user's individual pattern of menstruation and daily BBT readings. It progressively calculates more personalised green (infertile) days during the follicular phase, using the earliest recorded ovulation day as its reference point.

Identifying Infertile Days After Ovulation

• Traditional NFP: BBT is considered to have risen when three consecutive readings are all higher than the previous six days, with the third value at least 0.2°C above the highest of those six (the "3-over-6 rule"). Two exceptions apply:
  1. If the third elevated value is less than 0.2°C above the previous six-day high, a fourth elevated day must be observed (which doesn't need to reach the 0.2°C threshold).
  2. One lower value among the three elevated readings can be disregarded — provided the overall third elevated value is still at least 0.2°C above the pre-rise average.
• Daysy: Daysy also uses BBT to determine post-ovulatory infertile days, but rather than applying fixed rules, it continuously compares each day's reading against statistically determined reference values. If a significant rise of at least 0.2°C is detected at the expected time of ovulation — or if BBT remains elevated for at least two consecutive days — Daysy concludes that ovulation has occurred and marks subsequent days as infertile. Every calendar day is individually assessed against prior values, rather than applying a fixed three-day window. Daysy then waits for the user to log the start of her next period — or, if the elevated phase extends unusually long, flags a possible pregnancy. If data is unclear or a cycle irregularity is detected, Daysy displays a yellow light, indicating that fertility status is temporarily unknown. Yellow days should always be treated as potentially fertile.

Cervical Mucus

The word "symptothermal" reflects the fact that NFP combines temperature with at least one other body sign — most commonly cervical mucus, or sometimes cervical position.

• Traditional NFP: In the days approaching ovulation, cervical mucus transitions from thick, sticky, and yellowish to clear, stretchy, and egg-white-like — a sign of peak fertility. Once the mucus returns to a thicker, darker consistency, the post-ovulatory phase can be assumed. Confirmation requires one to several days of optimal mucus quality, followed by three days of declining quality.
• Daysy: Cervical mucus is a valuable fertility marker for women experienced in observing their body signs, and the DaysyDay app includes an optional field to record mucus observations. However, because mucus appearance varies significantly between individuals — and because even NFP-trained participants often report uncertainty in interpreting it — mucus data is not incorporated into Daysy's algorithm. Daysy was designed to be accurate and easy to use without requiring specialist training, minimising the potential for human error.

From a scientific standpoint, there is currently no standardised digital method for objectively assessing cervical mucus quality. Factors including medications, dehydration, alcohol, and caffeine can all alter mucus consistency, making it a complex marker to interpret reliably in everyday use.

Cervical Position

Changes in the cervix itself — its position, firmness, and openness — can also provide clues about fertility. During infertile phases, the cervix sits lower and feels firmer; as ovulation approaches, it rises, softens, and opens slightly.

• Traditional NFP: Regular self-examination throughout the cycle allows women to track these changes and identify the fertile window.
• Daysy: Cervical position is highly individual and changes with childbirth and menopause, making it difficult to standardise. As with cervical mucus, there are no universal benchmarks — so cervical position is not factored into Daysy's calculations.

Other Key Differences

One significant advantage of Daysy over traditional NFP is its use of historical cycle data. Rather than evaluating each cycle in isolation, Daysy incorporates data from past cycles into its calculations — making it better at predicting the fertile window before ovulation occurs, which is particularly useful for couples trying to conceive.

A recent independent study found that only 0.6% of days indicated as infertile by Daysy were false negatives — meaning they fell within the actual fertile window. This reflects the precision of Daysy's algorithm when applied consistently.