Charting your basal body temperature

Woman looks at thermometer to find out her basal body temperature

Trying for a baby? Charting your basal body temperature can help you work out when you're ovulating, and therefore at your most fertile.

Your basal body temperature (BBT) rises slightly just after ovulation, so measuring and recording it each day should give you a picture of the point at which you are ovulating in your cycle. Basal body temperature charting is best used alongside other methods, because while it helps you build up a picture of your cycle, it’s not 100% reliable on its own. Also, it’s quite good fun, particularly if you’re a fan of a new notebook.

What is your basal body temperature?

BBT simply means ‘the lowest temperature of your body at rest’. It remains about the same for most of the month, but drops slightly just before ovulation and then spikes just after ovulation by 0.25 to 0.5°C. The temperature then remains high until your period arrives.

Basal body temperature kit

In order to test your BBT, you’ll need:

  • a digital BBT thermometer (an ordinary one won’t show the small increases you’re watching for)
  • an alarm clock to wake yourself up at the same time each day (you can go back to sleep after on a weekend, but don’t eat, drink or get up before taking your temperature)
  • something to record temperatures in. The thermometer often comes with a little book for the purpose, but otherwise you can draw yourself a graph in a squared notebook.

Keep your kit in your bedside drawer or somewhere you can reach it without moving too much.

How do I chart my basal body temperature?

Decide on a method

You can take your temperature orally, vaginally or even anally. Apparently the oral method is slightly less accurate, but it’s a drawback the majority of women are happy to accept in return for not having to stick a cold thermometer anywhere more intimate first thing in the morning.

Whichever method you choose, you need to stick to it for the whole cycle (and probably get more than one thermometer if you’re going to swap methods down the line…).

Get a routine – and an alarm clock

You need to take your temperature at the same time each day, so pick a time you can stick with. The time you’d usually get up on a weekday is a good idea; that way, if you have a lie-in on the weekend, you can set your alarm, take your temperature and go back to sleep (or wake up your partner if it looks like it might be ovulation day).

“Temping seems like a lot more hassle than it actually is. All you need is a regular waking time and a thermometer. The rest is just data inputting. Aside from it helping you to conceive it's very reassuring (and interesting) to know your body does all this stuff it's supposed to.”

temperature chart

Start tracking

On the first morning after your period starts (so probably cycle day 2) take your temperature when you wake at your chosen time. Make a note of it and plot it on your graph if you like – a graph gives you a really clear picture of what’s happening when. Take your temperature at the same time every day.

Keep an eye out for the thermal shift

If you notice a rise of 0.2°C compared with the last six days, try to have sex that day. The rise means you’ve already ovulated so it’s worth a go that day, but next month you’ll be able to use this information to have a more educated guess about when ovulation is likely to happen and ensure you’ve had plenty of sex in those all-important preceding days when you have the best chance of conceiving.

“You cannot take any individual day's reading in isolation; by themselves they mean nothing. You must take a view over several days before you can build a picture. My average normal temperature is about 36.2°C. When I ovulate my temperature rises to 36.5°C and can go as high as 37.0°C.”

Analyse your chart at the end of your cycle

It took till my fourth cycle of charting to really get the hang of it and see a clear ovulation pattern. I found a Fahrenheit thermometer helped, but really it was the consistency and gaining a better understanding of the process that helped.

Look for the point in your chart at which your temperature rose. This will be just after ovulation, meaning you probably ovulated on this day or the previous one. Now you know when in your cycle you ovulate, you can have sex on that day, but more importantly during the few days just leading up to it. That way when the egg leaves the follicle there will be lots of sperm hanging around waiting for it.

You should see that in the first half of your chart (your follicular phase) your temperature is slightly lower than in the second half (the luteal phase) as your temperature remains high until the end of the cycle (or, if you’re pregnant, until the placenta takes over and progesterone levels can drop again).

Your follicular phase might change in length a little from month to month, but your luteal phase should remain pretty much a constant, lasting an average of 14 days. If your luteal phase is consistently less than 12 days, speak to your GP, as this could be causing problems with conceiving and may need treatment.

Check your cervical mucus, too

Alongside recording your temperature, charting the changes in your cervical mucus will give you a clearer picture of what’s going on when. The mucus you produce just before ovulation is sometimes known as ‘egg white cervical mucus’ (EWCM) as you can stretch a strand of it between your thumb and forefinger and it has the appearance of raw egg white.

“I found temping (and observing other symptoms) made me much more familiar with my body, and I can now tell when ovulation is approaching.”

Find out more about the different signs of ovulation.