Well - so long as we're ovulating - women's bodies do the same thing every cycle.
After we’ve produced an egg (ovulated) our body temperature rises slightly, helping to keep the egg warm.
If our egg isn’t fertilised, it dies and our temperature only stays high until our next period starts. The day of our period, our temperature drops and our cycle starts all over again*.
Working out whether you're in the early low temperature part of your cycle or the later higher temperature part of your cycle helps you to know whether you're ovulating and when you're in the infertile part of your cycle.
OK, I’m curious. How do I take my temperature?
This will only work if you're not using hormonal contraception. First buy a suitable thermometer. An extra quiet one with a memory setting like this one is fine. There's no need to buy a specialist basal body temperature (BBT) thermometer. You just need it accurate to one decimal point (celsius). Anything else makes things more complicated than they need to be.
You need to take your temperature at roughly the same time every day (plus or minus 30 minutes) after at least 3 hours of sleep and before you get out of bed or have a drink. You simply grab the thermometer from a handy spot next to your bed - on your alarm clock or phone is a good option. Then put it under your tongue and lie there dozing for a minute or two (the hardship!).
When the thermometer beeps you can read and record your temperature straight away or you can turn it off and do this later in the day.
Am I infertile as soon as I have a higher waking temperature?
No! You need to be sure. This means waiting till you've had 3 temperatures that are at least 0.1 degrees centigrade higher than the previous six (the last one of them at least 0.2 degrees higher).
They don't have to be higher than all the previous temperatures in the cycle, only the previous six.
You are safe to have unprotected sex on the third morning of a higher waking temperature. If the third temperature isn't 0.2 degrees higher, wait another day. This is called the 3 over 6 rule.
Combine the 3 over 6 rule with noticing cervical fluid for maximum effectiveness. Temperature rise day should be after your last day of egg white cervical fluid. If there's still egg white about on your temperature rise day, wait another day.
What should my waking temperature be?
Before ovulation, waking temperatures usually range between 36.1 and 36.4°C (95 to 97.5°F). After ovulation, waking temperatures usually range between 36.4 and 37°C (97.6 to 98.6°F).
Temperatures that are consistently much lower or higher than this might indicate thyroid problems. See your doctor if you are concerned.
Will my temperature always rise on the same day of my cycle?
Once you've been taking your waking temperature for a while, you can see how much your temperature rise day changes from cycle to cycle.
Some women will find that there is very little variation - come hell or high water their temperature will always rise on day 17 or 18 of their cycle (which indicates that they ovulate about day 16 or 17).
Others will find that it changes. It is quite normal for good or bad stress to delay, or more rarely bring forward, ovulation (e.g. holidays or illness), exams, deadlines, moving house.
Regular and major variations in your temperature shift day - more than 7 days - is sometimes associated with an underlying health problem. You might find that lifestyle changes help, or you might need to see a doctor - see this book review for more info.
What if my temperature doesn't rise?
Sometimes you might find that you have a cycle where there is no obvious shift in temperatures from low at the beginning of the cycle to higher at the end of the cycle. Buy a new thermometer, make sure you're using it right and see what happens next cycle.
Another option for making sure your temperature is as accurate as possible is to take your temperature vaginally. I know, shocking stuff, but really it's no big deal. It's easier in lots of ways and means your temperature is less likely to be affected by mouth-breathing or changes in room temperature. And you can talk to your partner and the beeps are less noisy. (Yes, I am speaking from experience...)
What if I'm ill or have a bad night?
Fever, medication, a sleepless night and excess alcohol can all change your waking temperature. It is possible to make allowances for these things. Just make a note in your app or on your chart. You're looking for an overall pattern, a few off days here and there are OK - though they can be annoying if they are around the time your temperature is due to rise.
What if I forget or I take it at the wrong time?
If you forget just leave the box blank on your chart - don't join up the lines because that will make it harder to spot that you missed a day. It's good to get into a routine but don't beat yourself up about it. Your chart doesn't have to be perfect to be usable.
If you take it more than 30 minutes early or late, write the time you took it on your chart. The general rule is to take away 0.1 degree for each hour earlier that you take it and to add 0.1 degrees if it is an hour later. We warm up after our usual waking time.
Isn't there an easier way to take it?
There are all sorts of temperature-based devices out there, but at a price and some of them aren't available yet. The technology is still developing:
- Kindara have just released a thermometer called Wink. It vibrates rather than beeps, and communicates directly with the Kindara fertility app.
- Fertility monitors with thermometers include Lady-Comp and Cyclotest for both planning and avoiding pregnancy and DuoFertility and Ovusense for help with getting pregnant.
- There are also devices in development that mean you don't have to take your temperature at a specific time.Temp-Drop is a small object that you stick to the skin under your arm while you sleep. Ovuring is worn internally, like a vaginal ring. Both of these devices communicate with a phone fertility app.
Or you could stick with the tried and tested option...
I love that there's so much going on with fertility gadgets but I can't recommend any of them at the moment. It's not difficult to learn how to take your temperature and track your fertility (see the DIY page), and if you want to feel extra confident then get some specialist support from someone like me. Get in touch for a free, no obligation chat about whether fertility awareness is for you.
*For some women, the temperature drop doesn't happen until a day or two after their period has started. Your menstrual cycle starts on the first day of your period - proper bleeding, not spotting. Your cycle ends on the day before your next period starts.
**A temperature rise is a very strong sign that you have ovulated but it isn’t a guarantee. For example, some women have Luteinized Unruptured Follicle syndrome (LUF). This means there is a shift in temperatures but the egg stays stuck inside its little sac (follicle) and doesn’t reach the womb. Fertility treatment can help these women to get pregnant.