# Tracking Your Basal Body Temperature
When you start tracking your menstrual cycle, it’s always best to [[The Beginners Guide to Menstrual Cycle Tracking|start with the basics]]. However, after a while I recommend that you deepen your awareness of your cycle using advance tracking techniques in order to [[Why Ovulation Is the Key to Optimal Health|understand ovulation]].
These techniques include:
1. [[Observing Your Cervical Mucus|tracking your cervical mucus]]
2. tracking your basal body temperature
3. [[Monitoring Your Cervix|monitoring your cervix]]
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?|What is Basal Body Temperature?]]
> - [[#How BBT changes throughout the menstrual cycle|How BBT changes throughout the menstrual cycle]]
> - [[#How to track Basal Body Temperature|How to track Basal Body Temperature]]
> - [[#Determining ovulation|Determining ovulation]]
> - [[#Making sense of your temperature|Making sense of your temperature]]
> - [[#No changes in temperature|No changes in temperature]]
> - [[#Confirmation not prediction|Confirmation not prediction]]
> - [[#What factors affect your BBT readings?|What factors affect your BBT readings?]]
> - [[#Final Thoughts|Final Thoughts]]
## What is Basal Body Temperature?
%% [[basal body temperature definition]] %%
Basal body temperature (BBT) is defined as "the lowest natural, non-pathologic body temperature recorded after a period of rest".[^1]
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.
%% [[BBT one of the simplest and least intrusive methods to detect ovulation]] %%
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.[^2]
## How BBT changes throughout the menstrual cycle
![[basal body temperature.excalidraw.svg]]
%% [[BBT is lower before ovulation]] %%
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℉). And it remains on this lower side until approximately 1 day before ovulation, when BBT reaches its lowest point. This is also known as 'nadir', or dip.[^2]
%% [[the rise in BBT is due to the thermogenic effect of progesterone]] %%
After ovulation, progesterone is produced during the luteal phase. Progesterone has a thermogenic effect which means that it is heat inducing. %% [[BBT increases after ovulation]] %% 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.
%% [[BBT returns to lower range just before menstruation]] %%
Then just before the onset of menstruation, progesterone levels fall which consequently lead to the basal body temperature returning to the lower range. The BBT returns to the lower range within 1–2 days before, or just at, the onset of menstrual bleeding.
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
%% [[BBT thermometer]] %%
Firstly you need to get yourself a [[BBT thermometer|BBT thermometer]] that measures to 2 decimal places (Eg 37.36℃).
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. [^3]
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.[^4] She says that this is to ensure that you have enough time for your resting metabolism to 'reset' itself.[^5]
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. [^6]
Then record your temperature in an app (which is the easiest way) or on a paper chart.[^7] Again this is a personal preference. Some people like apps. Some like to [[Free Menstrual Cycle Tracking Templates & Guide|keep a paper chart]] by their bed. It’s entirely up to you.
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
%% [[detecting ovulation with BBT]] %%
There are a few different ways to determine 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".[^2] 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.[^2]"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 [[Fertility Awareness Mastery Charting Workbook by Lisa Hendrickson-Jack|can make the results more consistent]].
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.
%% [[monophasic bbt does not mean that you have not ovulated]] %%
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.[^8]
%% [[BBT correlated with ovulatory hormonal pattern in 70% of subjects]] %%
%% [[20% of ovulatory cycles showed a monophasic BBT]] %%
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).
%% [[10% of cycles showed anovulatory or deficient luteal phase]]%%
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.
%% [[low bbt could indicate a thyroid issue]] %%
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 out.[^9]
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.[^2]
%% [[factors influencing BBT]] %%
Factors influencing BBT include:
- 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.
[^1]: [[(Steward and Raja, 2022)]] Steward, K. and Raja, A. (2022) ‘Physiology, Ovulation And Basal Body Temperature’, in StatPearls. Treasure Island (FL): StatPearls Publishing. Available at: [LINK](http://www.ncbi.nlm.nih.gov/books/NBK546686/)
[^2]: [[(Su et al., 2017)]] Su, H.W., Yi, Y.C., Wei, T.Y., Chang, T.C., Cheng, C.M., 2017. Detection of ovulation, a review of currently available methods. Bioeng Transl Med 2, 238–246. [LINK](https://doi.org/10.1002/btm2.10058)
[^3]: I started orally but have moved to axillary as I found holding my thermometer in my mouth was hurting my jaw.
[^4]: [[Fertility Awareness Mastery Charting Workbook by Lisa Hendrickson-Jack]]
[^5]: Dealing with disruptions and disturbances to sleep with BBT readings...Please don’t think your need to wake up early on weekends. After all, the benefits of sleeping really does outweigh the benefits of tracking your cycle. As Lisa Hendrickson-Jack points out, cycle tracking fits into your life and not the other way around.
[^6]: I aim to take mine at 7am which means that I give myself some leeway and usually take it between 6am and 8am.
[^7]: If you record your temperature on a paper chart you round up if it is 0.5 or higher and round down if it is 0.4 or lower.
[^8]: [[(Johansson, Larsson-Cohn and Gemzell, 1972)]] Johansson, E.D.B., Larsson-Cohn, U. and Gemzell, C. (1972) ‘Monophasic basal body temperature in ovulatory menstrual cycles’, American Journal of Obstetrics & Gynecology, 113(7), pp. 933–937. Available at: [LINK](https://doi.org/10.1016/0002-9378(72)90659-X).
[^9]: [[(Gustafson, 2015)]] Gustafson, C. (2015) ‘Denis Wilson, md: Low Body Temperature as an Indicator for Poor Expression of Thyroid Hormone’, Integrative Medicine: A Clinician’s Journal, 14(3), pp. 24–28. Available at: [LINK](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4566469/)