Why take your temperature?

When a woman ovulates (or releases an egg) her temperature rises slightly and stays elevated for at least 10 days. Taking your temperature daily can reveal this pattern to show you when your body tends to do this. As a guide, ovulation usually occurs 14 days before the next period is due to arrive.

Temperature taking does not PREDICT ovulation because the rise in temperature only occurs once the egg has been released. However, it does help confirm that you are actually ovulating and give you an idea of the timing in relation to the rest of your menstrual cycle.

Be aware that it is not the actual temperature in itself that is important, but how it changes each day over the menstrual cycle, . Write your temperature each day on your fertility chart by drawing a small dot at the temperature level on the relevant day of your cycle. You then connect-the-dots from day to day to show a lined pattern of your temperature over the month. When you ovulate, the temperature will often show a rise upwards in your line of connected dots, staying there for at least 10 days and then dropping a few days before a period arrives. If it stays elevated and no period arrives, you are probably pregnant.

How to take your temperature

The temperature used to monitor a woman’s fertility is called Basal Body Temperature or BBT. This is defined as the temperature of a healthy person on waking each morning. To record your temperature changes accurately, you will need a thermometer that has readings of small temperature changes ie. 10 levels between each degree. For example 37.1 to 37.9oCelsius or 98.1 to 98.9oFahrenheit. Electronic thermometers are OK if you follow the manufacturer’s instructions. Mercury thermometers need to be left in place for at least 3 minutes to record the correct temperature. You need to take your temperature every morning, as soon as you wake and before you get out of bed (after at least 3 hours sleep).

To make the reading more accurate you cannot eat, drink (or smoke) before taking your temperature. You can place the thermometer in your mouth (orally) or into your vagina. However, vaginal temperatures will always be a little higher than oral temperatures so if you start taking your temperature using one method (for example orally), you will need to continue taking it the same way every day for that month (until your next period arrives).

Some factors can make your temperature higher or lower than it would normally be, distorting the pattern. This does not mean you are not ovulating, but it may be harder for you to detect when it is occurring.

These can include:

• Being unwell. If you are ill and have a fever you will experience a temperature rise not related to ovulation. Although it is usually a significantly higher temperature if you are unwell, compared to the slight rise with ovulation.
• Travelling, climate and time changes can all affect your normal body temperature.
• Feeling unusually stressed can disrupt your body and perhaps your basal body temperature.
• Doing shift work can interfere with your body’s biorhythms and cyclical temperature changes over the day.
• Having interrupted sleep. You need at least a good 3 hours of sleep prior to taking your basal body temperature.

You can still keep taking your temperature when these factors are happening for you, but you need to make allowances for them when interpreting your fertility chart. Make a note of them in the “cond. affecting temp.” column if and when they occur. You can use your own lettered code, such as ‘s’ for sick, or ‘t’ for travelling.

NOTE: Be aware that it may take a few menstrual cycles before a typical pattern is obvious and ovulation can be estimated in an accurate way.