In today’s modern era of smart technology, it’s easy to forget that our bodies offer a wealth of information about our health if we know how to listen and interpret the data. Temperature monitors and charting apps make collecting and analyzing the 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; and “resting” is typically defined as a rested or sleeping state of at least three hours. When monitored daily over the course of the menstrual cycle, a pattern unfolds that gives an indication of ovarian hormonal activity, making BBT a major sign of fertility. During the preovulatory phase, estrogen is the dominant ovarian hormone and keeps your core temperature at a lower state.
Progesterone dominates the post-ovulation phase of the menstrual cycles 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 that measures your temperature to 1/100th of a degree (two decimal places) or, our personal favorite, Tempdrop.
Note that BBT thermometers are not your standard fever thermometer and are specially made to take temperature to two decimal places. But, collecting one data point per day with a BBT thermometer can result in “noisy” data if the following guidelines aren’t followed:
- Waking temperature needs to be at the same time each day (set an alarm!)
- Requires at least three hours of uninterrupted sleep
- Co-sleeping or snuggling can increase body temperature
- Alcohol consumption the previous night can elevate your waking temperature
- Fever due to illness will muddle your thermal shift
The benefit to Tempdrop is that it vastly simplifies the process of charting temperature. It will learn and precisely pinpoint your personal nightly and monthly 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, we officially recommend not wearing Tempdrop when you have a fever.
What is a Thermal Shift?
A thermal shift is the 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.
How to Interpret the Data
It’s time to get a little technical. To clarify chart reading and divide the pre-shift temps from the post-shift temps, you need to establish a coverline. It is important to know the why behind the how of drawing a coverline. 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.
Sensiplan Rules (NeoFertility)
According to Sensiplan, the minimum rise in temperature to detect a thermal shift is 0.05C/0.1F and the coverline goes through the highest of the previous six temperatures once a shift is detected.
Exception #1 Weak Thermal Shift
First, second, and third temps must be at least 0.05C/0.1F above the coverline. If the third temp is not at least 0.2C/0.4F above the coverline, wait for a fourth temp at least 0.05C/0.1F above the coverline.
Exception #2 Fallback Shift
The first temp must be at least 0.05C/0.1F above the coverline. If the second or third temp is on or below the coverline, the third temp must be at least 0.2C/0.4F above the coverline
Bringing It All Together
It is 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. If you’re curious about cervical mucus, you’re in luck, we'll be publishing a full blog post on it in the next few weeks!
If you'd like to learn more about FABM, including how to chart your cycle, identify your fertility window, and be your own ovulation calendar, download Tempdrop's free Introduction to Fertility Awareness, a beautifully-designed guide written by seven certified educators.
By Melissa Buchan, Founder of Virtus Fertility Care, LLC, Neo Fertility Advisor, CFCP (Certified Fertility Care Practitioner)
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.