Fertility-Awareness (based) Methods
The term "fertility awareness" combines different methods by which women observe their physical symptoms (such as basal body temperature and cervical mucus) to determine their so-called "fertile window " (1). All fertility awareness methods are based on the same two principles:
- Women can only become pregnant if they have unprotected sexual intercourse within this "fertile window" - i.e. a maximum of 5 days before ovulation).
- Women usually only ovulate once per cycle. However, if they ovulate more than once, the second ovulation takes place within 24 hours of the first one (2).
The length of the fertile window is defined by the maximum lifespan of sperm in the uterus (up to 5 days), and the lifespan of the unfertilized egg (up to 24 hours after ovulation). (2)
What fertility awareness methods are there?
There is a fundamental difference between the main methods: On the one hand there is the purely statistical calculation of the fertile window that is used by the calendar method, and on the other hand we have the methods that also analyse physiological symptoms, such as basal body temperature and cervical mucus, which help indicate whether or not you are fertile in your current cycle.
Measurement of basal body temperature
The physiological basis for the measurement of the so-called waking up temperature, often abbreviated as BBT (Basal Body Temperature), is that your body temperature rises slightly approx. 24 hours after ovulation. This increase in BBT is caused by the hormone progesterone and temperatures remain at a constant high level (approx. 0.2 °C above the average temperature level of the follicular phase) until the onset of menstruation. Responsible for this effect is the follicle membrane, also known as the corpus luteum, which changes into a gland after ovulation and then releases this hormone. Among other things, progesterone ensures that your body prepares itself for a possible pregnancy - just like a "nest" that is warmed up like an incubator. The same hormone also ensures that no further ovulation can take place. This method makes it possible to determine whether ovulation has taken place or not with almost 100% certainty (3). If the ovum was not fertilised, temperatures will drop towards menstruation and a new cycle begins.
Observation of the cervical mucus
While the change in the average basal body temperature is caused by the hormone progesterone after ovulation, the consistency of the cervical mucus is influenced by the hormone estrogen in the phase before ovulation. As oestrogen rises, which is released by the maturing follicles a few days before ovulation, the cervical mucus liquefies to facilitate the sperm's path for possible fertilization of the egg. By observing and accurately interpreting this estrogen parameter, it is possible to narrow down the fertile window relatively precisely in order to use these days successfully for baby planning (4). If you would like to learn more about cervical mucus analysis, you can .
The calculation of the fertile window is based on the average length of the previous cycles. It is assumed that the luteal phase, the second half of the cycle after ovulation, is always 14 days long. The fertile window opens four days before and closes three days after the predicted time. Information from the current cycle is not taken into account in this type of calculation. Since ovulation and thus the individual fertile window can fluctuate by an average of 5 days within a year, this method is at best recommended for general observation (5) and not nearly as reliable as the two methods described above.
In order to increase its accuracy, different methods are often combined, whereby mainly these two combination methods have been successfully established in the past three decades:
- The sympto-thermal method, as the name suggests, determines the beginning and end of the fertile window using a symptom and a thermal effect. In other words, the observation and exact interpretation of the is used together with the daily measurement and evaluation of the basal body temperature. Different rules are used to best define the fertile window. Since various factors can influence both the basal body temperature and the consistency of the cervical mucus, this method requires special training for precise application (6), its accuracy highly depends on the user and her knowledge.
- With the calculothermal method, it is assumed that all days after menstruation could be potentially fertile due to the natural variability of the female cycle. On the basis of this information, infertile days after menstruation are calculated in the coming cycle, always starting from the shortest follicle phase (between menstruation and ovulation) and the knowledge that sperm can survive up to 5 days inside the female body. By combining basal temperature and statistical calculation of infertile days after menstruation, it is possible to respond individually to the natural variability of the cycle with maximum accuracy (3).
What method does Daysy use?
We want Daysy to be easy to use for any woman without having to have any knowledge of cervical mucus or cycle analysis. Our goal was to create a device that is easy to use and reliable, without additional uncertainties such as a possibly misinterpreted cervical mucus consistency. Daysy is based on the calculo-thermal method. She combines the collection and learning of new data (your daily basal body temperature, beginning and end of menstruation, and accumulated past cycle data) and statistical methods (e.g. temperature rise after ovulation) that allow predictions of the coming cycle. This makes it possible to individually calculate infertile (green) days after menstruation.
You can use the DaysyDay app to personally note the nature of your cervical mucus. However, seeing that there is no standardized, reliable method to objectively observe and evaluate the mucus yet - especially considering the various factors that can affect the nature of the mucus - those personal notes will not be used by Daysy for her calculations.
Daysy is an intelligent fertility tracker that lets you get to know your very own menstrual cycle.
1. CDC - Fertility Awareness–Based Methods - USMEC - Reproductive Health. Available at: (Accessed: 1st April 2019)
2. Wilcox, A. J., Dunson, D. & Baird, D. D. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ 321, 1259–62 (2000).
3. Prior, J. C., Naess, M., Langhammer, A. & Forsmo, S. Ovulation Prevalence in Women with Spontaneous Normal-Length Menstrual Cycles – A Population-Based Cohort from HUNT3, Norway. PLoS One 10, e0134473 (2015).
4. Pyper, C. M. Fertility awareness and natural family planning. Eur. J. Contracept. Reprod. Health Care 2, 131–46 (1997).
5. Johnson, S., Marriott, L. & Zinaman, M. Can apps and calendar methods predict ovulation with accuracy? Curr. Med. Res. Opin. 34, 1587–1594 (2018).
6. Raith-Paula, E., Frank-Herrmann, P., Freundl, G. & Strowitzki, T. Natürliche Familienplanung heute. (Springer Berlin Heidelberg, 2013). doi:10.1007/978-3-642-29784-7
Autoren: Niels van de Roemer, Andrea de Groot, Petra Schenke