What is basal body temperature?
Your basal temperature simply means ‘the lowest temperature of your body at rest’. For most women, an average BBT is around 36.1 to 36.4°C. This temperature remains about the same for most of the month, but drops slightly just before ovulation and then spikes just after ovulation. The temperature then remains high until your period arrives.
The increase and decrease in temperature around ovulation is as slight as 0.25 to 0.5°C. For this reason, BBT charting is best used alongside other methods when trying to conceive (such as ovulation predictor kits), because while it helps you build up a picture of your menstrual cycle, it’s not 100% reliable on its own. That said, it’s quite good fun – particularly if you’re a fan of a new notebook.
Do you know when you're ovulating? Try our ovulation calculator
How do I chart my basal body temperature?
In order to measure your BBT, you’ll need:
a digital basal 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 this 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.
1. Decide on a method
You can take your temperature orally, vaginally or even anally. Apparently the oral method gives a slightly less accurate reading, 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…).
2. 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).
“Temperature charting 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.”
3. Start tracking your basal body temperature
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.
“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 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.”
4. Analyse your BBT 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.
At the end of a month, look back over your temperature chart. You should see that in the first half of your cycle (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.
5. Identify the best time to conceive
Look for the point in your chart at which your BBT 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 leading up to it. That way when the egg leaves the follicle there will be lots of sperm hanging around waiting for it.
In the first month when you are charting your temperature, if you notice a rise of 0.2°C compared with the last six days, try to have sex that day. 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.
Charting your cervical mucus
"I found temperature charting and observing other symptoms made me much more familiar with my body, and I can now tell when ovulation is approaching."
Alongside recording your temperature, tracking 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.
Find out more about the different signs of ovulation.