Charting your basal body temperature (BBT) for several months can confirm whether you’ve ovulated or not, and may help you determine the days of your cycle that make up the fertile window. However, this method does require a lot of diligence, consistency and patience every morning so can be a less reliable method in predicting ovulation.
What is basal body temperature?
BBT is the temperature when you’re resting. Normal body temperature is around 36.5°C (97.0-97.5°F). About 12-24 hours after you ovulate, your BBT increases slightly to 36.5-37.0°C (or 97.6-98.6°F) and stays at this level for the rest of your cycle until your next period. So if your BBT increases slightly on cycle day 16, then you likely ovulated on cycle day 14 or 15.
This increase in BBT is due to the increase in a hormone called progesterone which is released into a woman’s system after ovulation. Progesterone helps build the uterine lining to prepare the uterus for successful implantation of a fertilised egg and also supports other functions to prepare the body for pregnancy.
Which thermometer is best?
It’s best to use a special thermometer that can measure temperature in 1 or 2 decimal points, such as a basal or ovulation digital thermometer. This is because changes to BBT across your cycle will be very small so you need a thermometer that is sensitive to these changes. Most digital thermometers, including ones for babies, measure 1 decimal point (e.g. 0.1°C or 0.1°F). Thermometers can be purchased at pharmacies without a prescription.
How do I take my temperature?
This depends on your preferences and type of thermometer. The manufacturer should have clear instructions for use included with the product so read their leaflet carefully before using the thermometer. For example, the Surgipack Ovulation Digital Thermometer (A$13) can be used orally (by placing the probe under the tongue), rectally (by placing the probe inside the rectum) or under the armpit. Make sure to use the same method of testing each time.
Your BBT needs to be measured as soon as you wake up in the morning before getting out of bed or moving around.
Follow the tips below for taking accurate BBT measurements.
- Keep your thermometer within arm’s length will help you remember to measure your BBT each morning before you get up.
- Use the same thermometer and method (e.g. mouth, underarm) throughout your cycle.
- Try to measure your BBT at the same time each day (within 30 minutes of the usual is OK)
How can I chart my temperature?
There are many free, printable templates you can get online, or mobile apps. I prefer to chart using apps and am currently using Ovia where I can also include cervical mucous changes, ovulation test results and other symptoms (e.g. acne, bowel movements) throughout my cycles.
Try at least BBT charting for 3 cycles before attempting to interpret patterns.
Below is my BBT chart from my last cycle which was 28 days. My BBT increased on cycle day 16, so my ovulation day was likely on day 15.
Is BBT charting accurate?
If you have regular cycles and strictly measuring temperatures as recommended then BBT is reliable and accurate.
However, there are many factors that influence your BBT, such as:
- How consistently you measure your BBT every day, e.g. at the same time each day before moving too much
- Stress or illness
- Room temperature (e.g. hot or cold bedrooms)
- Your sleep pattern.
These factors can make accurate charting tricky and difficult which can make BBT charting less reliable. Also, BBT charting can only tell you when ovulation has already happened, it can’t tell you when you’re about to ovulate so unfortunately it may not be as helpful as other methods if you’re trying to get pregnant fast.
My personal experience with BBT charting
Although I consider myself disciplined and organised, I honestly found it difficult to measure consistently each day. On many mornings I forgot to test immediately after waking (it’s a bad habit but I always reach for my phone first thing!). I usually wake at 6:30am on weekdays but tend to sleep in on weekends to 9:00am, so I didn’t measure temperatures on weekends so there were a few holes in my charts.
This was my least preferred method to predict ovulation. It was reassuring to know that I was ovulating each cycle so I would recommend women who aren’t sure if they ovulate each cycle to try BBT charting for a few months. Speak with your doctor if you think you have an issue with ovulation or your menstrual cycles.
- Steward, K. & Raja, A. (2019). Physiology, Ovulation, Basal Body Temperature. StatPearls Publishing LLC. Accessed online: https://www.ncbi.nlm.nih.gov/books/NBK546686/
- Su, H. et. al. (2017). Detection of ovulation, a review of currently available methods. Bioengineering & Translational Medicine. Accessed online: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5689497/