What is Basal Body Temperature? (12b)

DEFINITION OF BASAL BODY TEMPERATURE (BBT) :

The basal body temperature or the waking temperature, is the temperature of the body at rest after at least three 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.

IS THERE A STANDARD METHOD OF RECORDING TEMPERATURE (BBT) ?

During the cycle the temperature is taken under the same conditions every morning, i.e. at the same time every morning, in the same orifice, using the same thermometer. Any change in conditions when taking the temperature is called a temperature disturbance and is recorded as such in the chart. (LINK to page 12d for temperature disturbances)

  • 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, and is called a temperature disturbance. 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. Persons on night shift should take their temperature in the evening on waking and before rising, after at least three hours sleep.
  • WHERE IS THEB TEMPERATURE USUALLY TAKEN ? :
    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 0.05 degrees C intervals, or if a Fahrenheit thermometer is used it should be marked at 0.1 degrees F intervals. Ear or forehead thermometers are not accurate enough for use, and neither are most oral thermometers. Your Natural Family Planning teacher would be able to provide you with a thermometer, or advise you where you can get one.

THE USUAL PROCEDURE IN TAKING AND RECORDING TEMPERATURE (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 0.05 degrees C and 0.1 degrees F. 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 36.64 degrees C, it is recorded in the 36.60 C box and not in the 36.65 C box.

To be most effective, the woman must be taught the symptothermal double-check method of Natural Family Planning by a qualified natural family planning teacher.