Basal body temperature (BBT) is your resting body temperature first thing in the morning, or after at least three hours of unbroken sleep. This means your temperature before you get out of bed, speak or have anything to eat or drink.

Why does it matter?

When an egg is released from your ovaries, levels of the hormone progesterone increase (if the egg is fertilized, it is progesterone that will prepare the lining of the womb to support the embryo). As levels of progesterone go up, so does your BBT. The change in temperature is only very slight (around 0.4 degrees and varying slightly from one woman to another) but it can nevertheless be an indication that you are ovulating.

Ovulation usually occurs around the middle of your cycle, though again, the length of a cycle varies from one woman to another. You are therefore at your most fertile during the few days before and after ovulation, as this is when sperm can reach and fertilise the egg.

If you take your BBT every day and keep a record of it, you may notice slight fluctuations in your temperature throughout your cycle. But unless it remains raised for three to four days, you are not ovulating. If your temperature remains raised for more than 18 days, it is likely that you are pregnant.

By keeping charts of your temperature every day for a few months you should start to notice a pattern. This will give you an idea of when you are most fertile so you can plan to try for a baby during those few days each month.

What is the best way to take my temperature?

Keep your thermometer by your bedside as you’ll need to take your temperature as soon as you wake up in the morning, before you get up and start your day. It’s best to take your temperature at the same time every day, as your BBT typically increases by around 0.1 degrees every half hour, so set your clock for the same time even at weekends (you can always go back to sleep afterwards).

Most BBT thermometers come with a chart. Make several copies so that you can chart your temperature for a few months to get an idea of when you’re likely to ovulate. Start at the beginning of your cycle (the first day of your next period) and keep a note of your temperature every day until the start of your next period, when you’ll start a new chart.

Which thermometer is best?

A special BBT thermometer is best as it gives a more precise reading. They are not widely available in chemists, but you can buy them online. You take the temperature in your mouth, as you would with a hospital thermometer. Make sure you clean it and put it back on your bedside table when you have used it so it is always ready the next morning.

Is this a good method of predicting fertility?

The temperature method is fairly simple to do in principle but you do need to be quite disciplined to get the most accurate results.

The results can be affected by other factors; if you have an illness you may get a fever, which will push your temperature up. It will also fluctuate if you have had alcohol or are taking painkillers
It is cheap as once you have the thermometer, the only other thing you need is a chart, which can be drawn on any paper.

This method will only show you when you have ovulated after the event. But by making a note of your temperature every day for several months, you should see a pattern developing.

This technique is most useful to women with very regular menstrual cycles. For some women, ovulation can take place at different times of the month, making it hard to see a pattern.

If you don’t take your temperature at the same time and when you first wake up each day you may not see a true pattern.