Understanding your Basal Body Temperature (BBT) Chart
- Carmen Fourie
- Apr 8
- 6 min read
Our Basal Body Temperature (BBT) is one of our primary fertility signs (there are two others). It is recommended to always use at least 2 of the 3 fertility signs for accuracy, but also that 1 of the 2 signs be our BBT.
In today's blog we'll have a look at how to understand our BBT chart & why it works as a fertility marker.
What is our Basal Body Temperature (BBT)?
BBT is a measure of your resting temperature. Which is why we measure it first thing in the morning after several consecutive hours of sleep (before doing anything else).
Interesting fact: Our BBT is a measure of our baseline metabolism, and our metabolism is a measure of efficiently our body transforms raw materials into vital energy to thrive. By taking your BBT you'll gain insights into your cycle & fertility, but you can also use it to gain insight into your metabolism. See the Fifth Vital Sign by Lara Biden.
Hormonal shifts in our cycle cause our BBT to go up or down. We can then measure our BBT to get an indication of where we are in our cycle.
What are we looking for with our BBT chart
Have a look at the image below - You will see there are temps below the line and above the line (that line is called a cover line).
In the chart above the jump in temperature from below the line to above the line serves as confirmation of ovulation and that you are in your luteal phase.
Luteal Phase: There are 4 phases of a cycle - menstruation, follicular phase, ovulation and luteal phase. The luteal phase is from immediately after ovulation until your bleed starts (first day of red blood, not spotting). It is typically 12 to 14 days. Our luteal phase is a non-fertile phase (pregnancy is not possible in this phase as long as you are very sure you have confirmed ovulation is over).
How we use our BBT to confirm ovulation?
Once we have ovulated our body transforms the follicle that ruptured to release an egg for ovulation into what is called a corpus luteum (hence the name luteal phase). This corpus luteum is a temporary cyst /glad that releases progesterone (the pro-gestation hormone because it is pro baby making and high in pregnancy).
Progesterone does lots of wonderful things (which is why ovulation is important even when women are not trying to conceive) and one of those things is that it increases our BBT (and our metabolism - which btw if why you may crave more food in your luteal phase). Your BBT should remain high for the duration of your luteal phase. The exception is that you may sometimes see a drop just before your bleed starts.
That means we can confirm ovulation and our luteal phase by looking for an increase in our BBT of around 0.2C or higher after 6 "low" BBTs. We want to see at least 3 raised BBTs before confirming ovulation is you are using fertility awareness to avoid pregnancy. Drawing a cover line like in the image below helps us to see the jump in BBT.
We say a jump after at least 6 "low" BBTs, but what this actually means is we want to see a jump after clearly having pre-ovulation and ovulation lower temps. You may have some outliers and therefore you won't see exactly 6 low BBTs, but you should see a clear pattern of preovulation and post ovulation temps with the BBTs after ovulation being clearly higher.
Outliers: Outliers are temps on your chart that are unusually high or low for the menstrual cycle phase, and the reason for them being unusually high or low is not related to the hormones in that phase, but rather related to something else like you being a bit ill (fever), or consuming alcohol, or lack of sleep, or stress, etc. You want to make a note if you know there is something other than your hormones impacting on your BBT.
Do you like the BBT chart in the image?
I'd love to come in here before I continue with more information to share that the BBT chart you see in the image is part of my Fertility Awareness Journal I created after many years of tracking my cycle and I think it is a really wonderful resource for fertility awareness.
You can find the printed version on Amazon and the digital version in my shop.
Counting our luteal phase days on our chart
Why would you want to do this? Unlike the other phases of our cycle, our luteal phase is typically fixed in number of days (12 to 14). That means if you confirm ovulation (using your BBT) and count your luteal phase days you will know when your bleed is expected to arrive.
For example, if someone has a long cycle this is usually because they ovulated later in their cycle - we do not necessarily ovulate halfway through our cycle.
After a few cycles you will know how long your luteal phase typically is. If your luteal phase is shorter than 11 days, it could be an indication of luteal phase deficiency.
If your luteal phase is longer than 16 days, it could indicate pregnancy. I, for example, did not take a pregnancy test at all when I was pregnant with my daughter and did not use a dating scan, yet I knew I was pregnant from my chart, and I knew the exact gestation period based off my chart (which was confirmed at my first scan at 32 weeks pregnant).
It is amazing what we can tell from our chart!
Back to counting your luteal phase days - your first raised BBT will be your luteal phase day 1. In my paper chart there is a space to specifically count the days. By counting the days after ovulation, you will know when your period is due and know if you actually "missed it" or not. Often when people think their period is "late" it could be because they didn't ovulate that cycle or they ovulated later in their cycle. The only true reason a period is late is pregnancy, but for that we need to know when our period is actually due based off our ovulation and not things like when our period usually comes.
The last day of your luteal phase is the day right before the first day of red blood. Spotting (brown or pink) is not considered your period.
What if you are not seeing a clear BBT chart of low and high BBTs?
There are many reasons for this - the most common is people not taking their BBT correctly. Literally 98% of the time people send me their charts and their BBT is a bit all over the place we narrow it down to them not taking their BBT correctly.
How to know if you are taking your BBT correctly?
That is a little beyond the scope of today's blog, but if you would like to deep dive into knowing your cycle, I co-created a course with a life coach and nutrition expert called COMING HOME OT YOURSELF. I created all the lessons on tracking your cycle, including details tips for taking your BBT correctly.
What is covered in the Fertility Awareness Section:
Cycle and Fertility Awareness & Appreciation
Introduction Discussion
What is Fertility Awareness
What does a healthy cycle look like
The biology of being a woman
What will you need to expand your awareness? - Introduction
Recording Information & Reflections
Free Hand Journal Prompts
What you need to measure your BBT
Expanding your awareness - A curious mindset
Fertility Markers Introduction
Fertility Marker - Basal Body Temperature
Fertility Marker - Cervical Fluid
Fertility Marker - Cervical Position
Secondary Fertility Markers
Ovulation Tests
Avoiding pregnancy - using your fertility awareness as a contraceptive
Trying to conceive
DEMO - Free Hand Journal
Living in sync with your cycle - A discussion with Sam
Additional Resources
Other reasons your BBT chart may not be clear
Aside from taking our BBT incorrectly, there are other reasons we may not have a clear BBT chart of low (pre-ovulation and ovulation) BBTs and high post-ovulation (luteal phase) BBTs. There are things like slow rising BBTs, hovering just above the cover line BBTs, and more. Usually what lies behind this is indicators of our health. By tracking your cycle, you will be able to gain so much insight and connection to your body!
I highly recommend my Fertility Awareness Journal as a resource for tracking and not just because I created it - I created it this way from many years of experience and experimenting with ways of recording details about my cycle and found that this works the best.
Physical Fertility Awareness Journal:
Let me know in the comments below if you have more topics around Fertility Awareness you want more information about - I appreciate you being here. Oh, and please sign up for my subscription.
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