What is Basal Body Temperature? - Definition & Examples

Instructor: Ashli Wilson

Ashli has a Master's Degree in Biology and has taught biology at different grade levels including college, elementary, and middle school.

In this lesson, we will learn about basal body temperature, how and why it is measured, what changes in basal body temperature can indicate, and how it is related to ovulation and thyroid function.

Basal Body Temperature

Both Nancy and Mike have decided to keep a daily track of their basal body temperatures. They have decided to keep track of their basal body temperatures because Nancy is trying to conceive and Mike thinks his thyroid gland's function has decreased. It may seem odd that measuring basal body temperature can help with determining the best time to conceive and decreased function of the thyroid but it does just that. Before we take a look at how basal body temperature can be used for conception and determining thyroid problems, let's understand what basal body temperature is.

What is Basal Body Temperature?

Basal body temperature is the lowest temperature of the human body, the body's at-rest temperature. Normally, basal body temperature is 97.8-98.2 degrees Fahrenheit. Keep in mind, ''normal'' varies from person to person. The key is consistency. Basal body temperature is measured after sleep but before any activity, as activity increases body temperature. You may wonder what the difference is between basal body temperature and normal body temperature. The difference is when and where each is measured. Also, body temperature can be measured orally or internally, while basal body temperature should be measured in the armpit because it is more accurate. Additionally, basal body temperature can only be measured after sleep and before activity, whereas body temperature can be measured at any time of the day.

How to Measure Basal Body Temperature

Basal body temperature thermometer

In order to measure basal body temperature, it's best to use a basal thermometer. A basal thermometer is more accurate than a regular fever thermometer. Also, a regular fever thermometer measures your temperature to the tenths place of a degree, whereas, a basal thermometer measures to the hundredths place to better track subtle differences.

To get an accurate reading, you need to get at least 3 hours of sleep to allow your body temperature to decrease. Then, upon waking, place the basal thermometer deep in the armpit for 10 minutes; observe the temperature on the thermometer. Keep track of your basal temperature by charting it each day. Most basal thermometers come with a basal temperature chart, a grid chart that allows you to easily graph your temperature to easily see any changes, such as spikes or drops. Again, the key is consistency.

Basal Temperature Chart
basal body temperature chart

Importance of Basal Body Temperature

Your basal body temperature is important because it can be used to predict fertility and determine productivity of the thyroid gland. Looking back at our example of Nancy and Mike, Nancy is tracking her basal body temperature because it increases after ovulation. For women, ovulation happens once a month and is the period of time when conception is most likely to occur. Women trying to conceive can measure their basal body temperature every day for a month and during the next month, use her basal body temperature to predict ovulation. Ovulation occurs 2-3 days before a woman's basal body temperature rises and by Nancy keeping track of her basal body temperature, she can predict the days she is ovulating in order to increase her chance of conceiving.

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