What Is Basal Body Temperature (BBT)?
Basal body temperature (BBT) is your body’s temperature when you are fully at rest. It is usually measured first thing in the morning, after several hours of uninterrupted sleep and before getting out of bed or doing other activities.
Hormonal changes during the menstrual cycle can affect BBT. After ovulation, progesterone may cause a small, sustained rise in temperature. Tracking BBT over time can help you notice patterns in your cycle.
How Basal Body Temperature Changes During the Menstrual Cycle
BBT varies from person to person and can change slightly throughout the menstrual cycle. After ovulation, many people notice a small temperature increase that remains elevated until the next period begins.
A consistent temperature shift over several days may indicate that ovulation has already occurred.
How to Track BBT
When to Measure BBT
Use a basal body thermometer, which is designed to detect small temperature changes. Take your temperature as soon as you wake up, before getting out of bed, talking, eating, drinking, or doing physical activity.
For the most useful results, take your temperature at about the same time each morning and use the same measurement method consistently.
What Can Affect a BBT Reading
Many factors can affect a BBT reading, including illness, fever, interrupted sleep, stress, alcohol use, travel, shift work, certain medications, and changes in your usual routine.
Recording these factors alongside your daily temperature can make it easier to understand changes in your chart.
What BBT Can and Cannot Tell You
Charting BBT over several menstrual cycles can help identify a pattern of temperature changes and may help you understand when ovulation has likely occurred.
Because the temperature rise happens after ovulation, BBT is most useful for confirming ovulation rather than identifying it in real time. If you are trying to conceive, tracking BBT may help you recognize patterns in your fertile window.
BBT tracking may be used as part of a fertility awareness-based method. However, BBT alone may not provide enough warning time to reliably prevent pregnancy, and it does not protect against sexually transmitted infections. Talk with a licensed medical provider if you have questions about fertility tracking or contraception.