What is the basal body temperature (BBT), and how is it recorded?

Definition of the BBT:

The basal body temperature or the waking temperature, is the temperature of the body at rest after at least 3 hours uninterrupted sleep. It is therefore recorded immediately on waking, before getting out of bed, and before any food or drink is taken or before smoking. The temperature is usually taken in the mouth, i.e. orally.

Time Taken :

The temperature is taken at the same time every day; this is because the temperature varies during the day, i.e. it is lowest in the early morning and rises gradually during the day. Therefore a temperature taken at a time later or earlier than usual, will be higher or lower respectively than if it had been taken at the usual time. Such a temperature has to be discounted when interpreting the temperature graph and in such cases the woman should seek the advice of her NFP teacher.

Oral or rectal temperature?:

The temperature can be recorded either by mouth (orally) or by rectum, but the same body location must be used every day as the temperature in the mouth is slightly lower than the temperature in the rectum.

Is a special thermometer required?:

As the rise in temperature is very small a special digital thermometer is used. The thermometer may be measured in either the Centigrade or Fahrenheit scale; if using a Centigrade thermometer the scale should be marked at 0.05ºC intervals, or if a Fahrenheit thermometer is used it should be marked at 0.1ºF intervals. Ear or forehead thermometers are not accurate enough for use, and neither are most oral thermometers. Your NFP teacher would be able to provide you with a thermometer, the cost of which is under 10 Euro, or advise you where you can get one.

The usual procedure in taking and recording the BBT:

The thermometer is placed under the tongue and left in place until it bleeps, (follow the instructions given by the manufacturers of the thermometer). The temperature is recorded on the chart each day by putting a dot in the centre of the square opposite the corresponding temperature on the chart and the dots are then joined to form a graph. The temperature chart is divided in intervals of 0.05ºC and 0.1ºF. If the temperature obtained is in between the numbers on the chart it is recorded as the nearest lower temperature. For example with the centigrade thermometer, if the temperature obtained is 36.64ºC, it is recorded in the 36.60ºC box and not in the 36.65ºC box.