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Understanding basal body temperature (BBT)

Your basal body temperature is as unique as yourself. One month it goes like this and the next month it goes differently. That is why it is useful to have some examples with which you can compare your temperature curve. It certainly does not have to be exactly as below because your hormonal balance determines your Basal Body Temperature (BBT). Daysy is there to determine and analyse your BBT.

The 'typical’ curve

typische curve van de menstruatiecyclus, basale lichaamstemperatuur, temperatuurmethode met Daysy de cyclus tracker

Here you can see a "typical" temperature curve (28 days). Ovulation took place between day 12 and day 13, recognisable by the significant rise in basal body temperature. The corpus luteum phase (after ovulation) lasts 15 days and ends with the onset of menstruation.

Drop in temperature before ovulation

temperatuurmethode nauwkeurig ovulatie bepalen, kans om zwanger te raken

As you can see in this sample curve, there are curves where the last value before ovulation (see arrow) is significantly lower than on most other days previously measured. The reason for this is increased oestrogen levels shortly before ovulation.

Good to know: this is the ideal day for sexual intercourse if you want to get pregnant.

Not every woman has a drop every month!

Increased average value

hogere basale lichaamstemperatuur

A relatively high average temperature, as seen in this sample curve, may indicate an over-functioning thyroid gland (hyperthyroidism).

Other symptoms of hyperthyroidism may include:

  • Long cycles

  • Little blood loss during menstruation, short periods

  • General restlessness and nervousness

  • Rapid, increased sweating

Despite the increased average temperature, the basal body temperature curve shows the typical picture with an increase after ovulation and a significant increase during the corpus luteum phase. Only a doctor can diagnose hyperthyroidism.

Low average value

lage basale lichaamstemperatuur, schildklier werkt mogelijk traag

This example shows morning temperatures lower than 36°C, which the DaysyDay app lets you know after syncing. Low temperatures below 35°C are automatically excluded from the algorithm and displayed as a yellow or red day if you are within your fertile period. These low morning temperatures may indicate under function of the thyroid gland a hypothyroidism.

Further indications of hypothyroidism may include:

  • Absence of menstruation or amenorrhoea

  • Persistent heavy menstrual periods

  • Irregular cycles

  • Cycles without ovulation (no temperature rise)

  • Short luteal phase

  • Temperature fluctuations

However, despite the low average temperature, the basal body temperature curve shows the typical pattern with a characteristic increase after ovulation and a significant increase during the corpus luteum phase.

Important: Whether you are suffering from thyroid dysfunction can only be determined by a doctor.

Temperature outlier

verloop basale lichaamstemperatuur is niet iedere cyclus hetzelfde

If a reading is clearly out of range (see arrow in the example curve), Daysy will ignore it, but in case of doubt a yellow or red day is indicated

Such one-off outliers can be caused by alcohol consumption, fever, too little or very long sleep or by an incorrect reading. More on factors affecting your cycle.

Illness and fever

Ziek of koorts dan meet je niet met Daysy de cyclus tracker

Fever is when your body temperature reaches at least 38°C - if this is the case, you are most likely sick and you will notice that your temperature curve is unusually high (see sample curve). The DaysyDay app alerts you that your temperature is high and recommends measuring with a standard medical thermometer. Temperatures above 37.8°C or below 35°C are automatically excluded from the algorithm and are not used for your fertility calculations.

General rule: if you wake up in the morning and feel different from normal, e.g. sick, hungover or overtired, skip the measurement that day and possibly the following days until you feel better again.


Start taking your BBT now!

Start with Fertility Tracker Daysy



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