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 fluid, 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 fluid
While the change in the average basal body temperature is caused by the hormone progesterone after ovulation, the consistency of the cervical fluid 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 fluid 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 fluid analysis, you can read more here.
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: