What Is Basal Body Temperature And Should You Be Tracking It Every Day When You Wake Up?

With the growing distrust of hormonal birth control, many women are looking for alternatives to manage their menstrual cycle or prevent pregnancy naturally. This means they have to start learning about their body and its biomarkers. There are certain things to look out for, such as cervical mucus and basal body temperature (BBT). You may have heard of BBT before, especially if you've been seeking a natural method to comprehend your fertility cycle. But if this term is new to you, you may want to learn more about this biomarker and why it can be useful for you to track it for menstrual health, fertility, and more.

What Is Basal Body Temperature?

BBT refers to the body's lowest temperature achieved during rest, typically taken immediately after waking up from a prolonged sleep. Unlike the temperature checks you might perform when you're feeling unwell, BBT is an indicator of your body's base metabolic temperature when it's in a state of rest. Because of the hormonal changes throughout a woman's menstrual cycle, BBT can fluctuate subtly, providing clues about where she might be in her cycle.

BBT refers to the body's lowest temperature achieved during rest.

"The reason we use it for charting is because it helps pinpoint ovulation in a full chart," fertility awareness educator Mairead Suthoff tells Evie. "BBT rises after ovulation, which tells us that progesterone is the dominant hormone, which is only produced after ovulation. Progesterone raises our basal metabolic rate and therefore our resting temperature."

During the follicular phase of the menstrual cycle, estrogen dominates, and BBT remains relatively low. However, once ovulation occurs, progesterone takes over, causing the noticeable increase in BBT that Suthoff describes.

"Pre-ovulation, our temperature doesn't tell us much, but post-ovulation, our BBT will elevate about 0.03 degrees Fahrenheit above our pre-ovulation temps and should stay elevated," Suthoff continues. "If it drops too much, that can indicate insufficient progesterone production or a false peak, meaning that ovulation didn't actually happen."

This temperature shift is an unmistakable sign that ovulation has taken place, giving women a more exact understanding of their fertility window. For those trying to conceive, this information can be vital, as the chances of pregnancy are highest during the few days around ovulation.

BBT tracking is not just for women looking to become pregnant; it's also a useful tool for those wanting natural, hormone-free contraception methods. Fertility Awareness-Based Methods (FABM), which include temperature monitoring, can be as effective as hormonal contraceptives when used correctly. BBT tracking allows women to identify "safe" days, where the risk of pregnancy is low, and "unsafe" days, where extra precautions are necessary.

BBT rises after ovulation.

Additionally, consistent tracking of BBT can flag menstrual health issues or irregularities, such as anovulatory cycles where ovulation does not occur, luteal phase defects, or possible early-stage pregnancy. For women with Polycystic Ovary Syndrome (PCOS) or other menstrual disorders, tracking BBT can be an integral part of managing their condition and planning for medical interventions.

Suthoff stresses that "BBT is useful as retrospective data." You can use the information to observe ovulation after it has happened, but it doesn't tell you when you're currently in your fertile window. "Combined with a biomarker like cervical mucus, it becomes much more useful, however, because you can see your entire fertile window," she explains. "Your fertile window is observed through cervical mucus changes, primarily, but you can also add data like BBT or LH strips (hormone strips to 'predict' ovulatory activity)."

How Do I Track Basal Body Temperature?

Tracking BBT requires consistency and a few simple tools. The first step is to acquire a basal body thermometer, which is more sensitive than regular thermometers and can measure slight temperature variations up to two decimal places. It is crucial to use the same thermometer throughout your tracking for accuracy.

Timing is key: Measure your temperature first thing in the morning, immediately upon waking, and ideally at the same time each day. It's important to have at least three consecutive hours of sleep for the most accurate reading. You should avoid eating, drinking, or any physical activity before taking your temperature, as these activities can affect the results.

BBT tracking is not just for women looking to become pregnant.

Once you've taken your temperature, record it in a tracking chart or an app specifically designed for BBT tracking. Many apps are available that automate this process, providing visual charts that make it easier to spot trends and shifts in temperature over time. Remember, even minor temperature fluctuations can indicate significant hormonal changes.

"Movement before taking your temperature is an issue because it will artificially increase your temperature, which can make it look like you may have ovulated when you didn't," Suthoff says. "This is especially true for women who rely heavily on BBT and don't incorporate other biomarkers like cervical mucus."

Alcohol consumption or having a fever from being sick can also artificially raise your temperature. "A poor night's sleep may have an impact on your reading also," Suthoff adds.

"I personally use and love the Tempdrop (LUMINA20 gets you 20% off) as a convenient wearable thermometer. It tracks your temperatures overnight and can detect if you're restless or if you've gotten up, which is great for moms or people with irregular sleep patterns," Suthoff says. "You can buy an inexpensive thermometer that reads to a 10th of a degree, and that is sufficient."

She also recommends the Tempdrop app (it's free); otherwise, she suggests the app called Read Your Body. "It's a fully customizable fertility awareness app," Suthoff says.

After tracking for at least one menstrual cycle, you should be able to identify the slight temperature rise that indicates ovulation has occurred. This is usually a rise of about 0.4 to 1.0 degrees Fahrenheit and will stay elevated until your next menstrual period starts. Spotting this trend over several cycles will give you a more accurate understanding of your fertility window.

Fertility awareness methods teach women how to observe their biomarkers.

"Overall, if you want to chart successfully for health or family planning, it's best to work with a certified instructor!" Suthoff says. "The information on charting is confusing because most of it is dated or skewed beyond recognition. What you see coming from the CDC, for instance, is false. Healthcare professionals lump all methods together under 'the rhythm method' usually, which is not what fertility awareness is at all."

Fertility awareness methods, which are different from the rhythm method, teach women how to "observe their biomarkers," such as cervical mucus, BBT, LH, etc. This helps women determine whether they are fertile for the current day they're tracking. "There is no prediction or assumption involved," Suthoff says. "Charting is extremely effective when done correctly."

The reason why so many women are turned off by it is because they simply don't have all the information they need to chart correctly. Working with an instructor can help alleviate confusion and make the process much smoother, especially for women who are coming off hormonal birth control.

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