By Clara Miret Lucio BSc, MSc (embryologist).
Last Update: 04/07/2014

When pregnancy does not occur, calculating the most fertile days of a woman is usually the first method used before undergoing an assisted reproductive technique.

There are easy home methods for this purpose such as charting your basal body temperature (BBT), which will be explained below.

What is the basal body temperature method about?

Basal body temperature (BBT) is described as the body’s temperature when we wake up.

The basal body temperature method is quite effective. However, combining it with ovulation tests is recommended, since there are factors affecting its accurate charting.

This method consists of recording the temperature every day after waking up by charting it. The vertical axis will display the temperature and the horizontal one will show the days of the cycle. Every day, after measuring the temperature, a dot will be written down within the chart until a line is formed.

Before ovulation, a woman’s basal body temperature may range steadily from 36.5°C up to 36.7°C.

To calculate the most fertile period of a woman, we will rely on the fact that two or three days after ovulation, her basal body temperature increases sharply between 0.3°C up to 0.5°C. This is caused by the progesterone hormone and it will remain high until the following menstrual period. The basal body temperature chart will enable us to see the day in which this sharp increase occurs. If there is variation between months, the chart presenting the lowest values will be used as a reference.

The very first day of low basal body temperature will be the first day of menstruation within the new menstrual cycle.

What can be used to measure the basal body temperature?

A thermometer is used to measure the basal body temperature. There are mercury-in-glass thermometers or digital thermometers:

  • Basal thermometer: it is the typical clinical, mercury-in-glass thermometer with a measuring range from 35°C up to 39°C. It should be under 35°C every night. In case you measure the temperature by mouth, you should wait for 5 minutes; conversely, it its measured through the vagina or rectum, the waiting time will be 3 minutes.
  • Digital thermometer: this thermometer is a little more expensive, although less fragile and safer. After 60 seconds, it will emit a beep alarm, meaning that the temperature can already be checked.

What time of the day and in what part of the body is the basal body temperature measured?

Temperature should be measured before getting up and after having rested for an hour or having slept at least 5 hours. In addition, it must be measured always at the same time each day. Alcohol, trouble sleeping, or stress may alter your basal body temperature. There are three ways of measuring your BBT:

  • Orally: put the tip of the thermometer under one side of the tongue and keep your lips together.
  • Vaginally: it is inserted into the vagina.
  • Rectally: after applying vaseline or lubricant, insert the tip of the thermometer in your rectum while you are in a bended position.
  • To avoid any variation, using the same way of measuring the BBT during the whole cycle is advisable.

    Simple and non-expensive are the most appropriate adjectives describing this method. However, using it as a form of contraception is not recommended, since it does not prevent to 100% pregnancy to occur, nor does it protect against sexually transmitted diseases.

Authors and contributors

 Clara Miret Lucio
BSc, MSc
Embryologist
Bachelor's Degree in Pharmacy from the University of Valencia (UV). Master's Degree in Biotechnology of Human Assisted Reproduction from the UV. Experience as a pharmacist and embryologist specialized in Reproductive Medicine at Valencian Infertility Institute (IVI), and currently at Equipo Juana Crespo (Valencia, Spain). More information