Tracking Your Basal Body Temperature

When you start tracking your menstrual cycle, it’s always best to start with the basics.¹ However, after a while I recommend that you deepen your awareness of your cycle using advance tracking techniques in order to understand ovulation.²

These techniques include:

1. tracking your cervical mucus ³
2. tracking your basal body temperature
3. monitoring your cervix ⁴

In this article I will discuss basal body temperature (BBT), what it is, how it changes throughout your menstrual cycle, and how to track it. I will also discuss how to determine ovulation using BBT, how to make sense of your temperature readings, and what factors may influence differences in your BBT.

What is Basal Body Temperature?

Basal body temperature (BBT) is defined as “the lowest natural, non-pathologic body temperature recorded after a period of rest.”⁵

In a nutshell, this means that your BBT is a measure of your resting metabolism. That’s it. It’s simply your body’s temperature. 

Now many of us already know that our temperature changes when we, say, have a fever. But what you may not know is that our BBT actually changes during our menstrual cycle too. And in actual fact, BBT one of the simplest and least intrusive methods to help us detect ovulation.⁶

How BBT changes throughout the menstrual cycle

When looking at our BBT, it is a story of 2 halves. Before ovulation, our BBT is on the lower side 36.11 to 36.67 °C (or 97.0 to 98.0℉).⁷ And it remains on this lower side until approximately 1 day before ovulation, when BBT reaches its lowest point.⁸

After ovulation, progesterone is produced during the luteal phase. Progesterone has a thermogenic effect which means that it is heat inducing. Hence a few days post ovulation, our temperature rises to 36.55 ℃ / 97.8 ℉ or above (usually) and remains at this elevated level throughout the luteal phase.

Then just before the onset of menstruation, progesterone levels fall which consequently lead to the basal body temperature returning to the lower range.⁹

This means that if you track your BBT and you notice that your temperature increases over a few days and remains elevated for the rest of your cycle, this may indicate that you have ovulated.

How to track Basal Body Temperature

Firstly you need to get yourself a BBT thermometer that measures to 2 decimal places (Eg 37.36℃).¹⁰ 

Once you have your thermometer, you will need to decide how you want to take your temperature. Orally is the most common, but you can also take it vaginally or axillary (under your arm). Whatever you choose is a personal preference. ¹¹

Whichever you decide you need to make sure you keep to the same way throughout a full cycle. If you choose to change how you take your temperature (like I did) then you will need to do it at the start of a new cycle so as not to mess up the readings.

You need to take your temperature first thing in morning. Preferably after 3 consecutive hours of sleep according to Lisa Hendrickson-Jack.¹² She says that this is to ensure that you have enough time for your resting metabolism to ‘reset’ itself.¹³ 

You also need to make sure that you take it before getting out of bed and before doing anything else as this can affect the reading. And try and take your temperature at the same time every day (where possible). I recommend a 2 hour window.¹⁴

Then record your temperature in an app (which is the easiest way) or on a paper chart.¹⁵ Again this is a personal preference. Some people like apps. Some like to keep a paper chart by their bed. It’s entirely up to you.¹⁶

You will also want to make a note of any events that could affect your BBT such as illness, alcohol or excess stress. Even if you know that your temperature is going to be out, track it anyway. It is never going to be perfect. You will always have a few dodgy looking temperatures in there. Just make a note of anything that may have affected your readings.

Determining ovulation

There are a few different ways to deterime ovulation using BBT. The first is using the the “coverline” method, whereby a horizontal line is drawn on a BBT chart. The coverline is determined by “adding 0.158F to the highest temperature recorded during the first 10 days of a cycle, or by using previously recorded temperatures.”¹⁷ Thus, when temperature above this line is recorded, ovulation is suggested.

A second way of determining ovulation is the “three over six” rule. This is where three temperatures are required to be higher than the previous six preovulatory temperatures.”¹⁸ Of course I am talking about “normal” temperatures so we need to make sure that these are not abnormally high dues to stress, illness or a poor nights sleep.

If you use an app, you can put your temperature into it and it will usually create a graph for you, helping you identify the changes easily.

I would also suggest that you don’t just use BBT alone to determine ovulation but combine this with monitoring your cervical mucus and cervical position.

Making sense of your temperature

Sometimes when we start tracking, temperatures can be a bit all over the place or we’re not sure what we are looking for. It does take time to get into the hang of so please, be patient.

You could also try leaving your thermometer in place for 10 minutes before pushing the button. This is called temping and can make the results more consistent.¹⁹

And again, make a note of any factors that could be affecting your recordings. But remember that the purpose of tracking is not to have everything perfect. The aim for us is body literacy and in identifying if and when we ovulate.

No changes in temperature

If you don’t see a change in your temperatures at all, it could mean a few things.

Firstly it’s important to note that a monophasic BBT (meaning that the temperature does not change) does not mean that you have not ovulated.

Research has found that some women do not see an increase in BBT despite ovulation occurring.²⁰

And in fact, another study showed that despite 70% of women had biphasic BBT which correlated with ovulation, 20% of women had a monophasic BBT (they did not see an increase in temperature post ovulation).

The same study also showed that 10% of women showed anovulatory or deficient luteal phases which could also mean that BBT does not show as we intend it to.

Secondly, if you do have consistency low basal body temperature it could also be an indication of a thyroid imbalance which you will want to get checked out²¹.

Either way, tracking your temperature means that you have the data to take to your GP which will give you a head-start in figuring out your next steps.

Confirmation not prediction

Remember that tracking your BBT only confirms ovulation, it does not predict it so it is definitely not a good biomarker to use on it’s own especially when it comes to baby-making or birth control.

However it does help predict when your period is coming. This is because the number of days between ovulation and your period is fairly consistent. 

So after tracking 3-6 cycles you will develop an idea of how long your luteal phase is. And so you can start to predict when your next period will be once you have confirmed ovulation.

What factors affect your BBT readings?

We need to be aware that there are certain lifestyle and environmental factors that can impact our basal body temperature readings. So if we want to be more accurate, we need to know what to look out for and to identify whether an increase in temperature is due to progesterone post ovulation or whether it’s due to other factors.²²

Factors influencing BBT include:

• fever
• alcohol
• stress (either emotional or physical)
• sleep disturbances
• shift work
• change of room temperature
• changes in climate or time zones
• recent start or discontinuation of certain medications
• drinking before you take the temperature (orally)
• changing how you take your temperature mid cycle (eg: from orally to vaginally)
• changing the thermometer that you use mid cycle

Final Thoughts

Basal body temperature (BBT) is a useful yet simple advanced method to track ovulation and monitor your menstrual cycle.

If done correctly, your temperature should increase slightly post ovulation due to the thermogenic effect of progesterone. However, if you are unable to detect changes in your BBT, it is worth investigating if there are any lifestyle or environmental factors that could be influencing your readings.

Additionally, if you are unable to detect any changes in temperature, it is worth speaking to your doctor to rule out the possibility of a thyroid imbalance or other possibilities.

Finally, it is important to remember that BBT is best used to confirm ovulation rather than predict it, and should always be used in conjunction with other methods such as monitoring cervical mucus and cervix position.