Written by Melissa Buchan
In today’s modern era of smart technology, it’s easy to forget our bodies offer a wealth of information about our health if we know how to listen to and interpret them. Temperature monitors and charting apps make collecting and analyzing these stats an incredibly simple task that we can easily incorporate into routines of daily living - and it’s essential to building a chart you can trust!
But, before we dive into the tech, tools, and apps, it’s important to have a solid understanding of why basal body temperature (BBT) is important and what it tells us about our fertility.
What is BBT?
BBT is your body’s resting temperature. “Resting” in this case is typically defined as a rested or sleeping state of at least three hours. When monitored daily throughout the menstrual cycle, a pattern unfolds giving an indication of ovarian hormonal activity, making BBT a primary fertility sign. During the follicular (pre-ovulatory) phase, estrogen is the dominant ovarian hormone and keeps your core temperature at a lower state.
Progesterone dominates the luteal (post-ovulatory) phase of the menstrual cycle and is responsible for a host of actions in the body, including elevated BBT. If we examine the word progesterone, it makes a lot of sense. “Pro” means before and “gest” refers to gestation... so progesterone prepares the body for gestation, or pregnancy, if conception occurs in any given cycle.
How to Take Your Temperature
The most common and reliable approaches to collecting your basal body temperature are with an oral, vaginal, or rectal BBT thermometer measuring your temperature to one hundredth of a degree (two decimal places) or, our personal favorite, Tempdrop.
Note that BBT thermometers are not the same as a fever thermometer. They are specially made to take temperature to two decimal places. The following information will help you keep your temperature measurements stable (rather than noisy) when you take it into consideration:
- Waking temperature needs to be at the same time each day (set an alarm!)
- Requires at least three hours of restful sleep
- Co-sleeping or snuggling can increase body temperature
- Alcohol consumption the previous night can elevate or lower your waking temperature
- Fever due to illness will increase your temperature
The benefit to Tempdrop is that it vastly simplifies the process of charting temperature. It learns to precisely pinpoint your personal nightly temperature patterns by monitoring and evaluating hundreds of data points each night and, because it’s worn overnight, it removes the need for alarm clocks!
Tempdrop still needs to record at least three hours of data and does not remove the need to evaluate questionable temps due to alcohol or illness, but it is able to deliver accurate data even if you wake up to use the restroom, tend to a sick child, or get a little frisky. Still, the official recommendation is still not wearing Tempdrop when you have a fever.
What is a Thermal Shift?
A thermal shift is a biological indicator that ovulation has taken place in a menstrual cycle. It provides a biphasic temperature pattern in a healthy ovulatory cycle. A general rule of thumb when evaluating temperatures is to look for a pattern of three elevated temperatures over the previous six lower temperatures. This “3 over 6” rule is a good standard to follow as you learn your individual temperature ranges. Each method has specific rules on how to determine a valid shift.
How to Interpret the Data
To clarify chart reading and divide the pre-shift temps from the post-shift temps, you need to establish a coverline. It's important to know the why behind the how of drawing a coverline. A coverline is an interpretation on your chart which helps you identify where the shift occurred and when ovulation is confirmed. Setting a coverline makes chart interpretation much easier at a glance, so you don't have to find where the shift occurred each time you look at the chart.
If data crunching is not your idea of a good time, consider using an app where your coverline is detected automatically. Just be sure you know what standards an app uses for coverline detection.
Bringing It All Together
It's impossible to predict exactly when ovulation will happen. A rise in temperature only indicates ovulation has passed and identifies post-ovulatory infertility. Cervical mucus is the most accurate indicator of the onset of the fertile window.
Charting BBT along with standardized observations of cervical mucus provides a highly accurate picture of both pre-ovulatory fertility and post-ovulatory infertility. To learn more about cervical mucus, check out our blog post all about it!
If you'd like to learn more about fertility awareness, including how to chart your cycle, identify your fertility window, and be your own ovulation calendar, download Tempdrop's free Starter Guide to Fertility Awareness, a beautifully-designed guide written by experts.
Hi, I'm Melissa. I teach women how to navigate their menstrual cycles for women’s health monitoring, natural birth control, and infertility evaluation. In 2006 I enrolled in a training program to become a fertility awareness instructor and I haven’t looked back.
Since then I have launched and grown several fertility awareness focused programs and this past year I had the great pleasure of co-hosting the inaugural Cycle Power Summit, an online conference educating women, couples, and the healthcare community on the power of fertility awareness.
I am also part of the dynamic Neo Fertility App development team. I am certified through the American Academy of FertilityCare Professionals, I operate an affiliate center of FertilityCare Centers of America, and I’m the first Neo Fertility Advisor in the United States. I am a member of IIRRM and FACTS.
My newest program, Chart Your Cycle, is a 5-week implementation program teaching busy women how confidently and accurately chart their menstrual cycle.