Measuring Basal Body Temperature with Fitness Trackers

What is Basal Body Temperature?

Basal body temperature (BBT) is the lowest natural body temperature recorded after a period of at least 6 hours of rest, usually in the morning after a full night's sleep. The most common use for BBT measurements has been for women to track their ovulation. Ovulation causes a natural increase in a woman’s BBT, which lasts throughout the remainder of the cycle until menstruation.

How is my basal body temperature measured?

To use BBT as a health indicator, it is measured every morning using an under-the-tongue thermometer or a fitness tracker. For the most accurate measurement, BBT should be measured at the same time every day, immediately after waking up. The BBT values can be tracked on a paper chart, or an app designed to chart BBT. Women will see a natural increase of 0.33 – 1.0 °F shortly after ovulation occurs.

What is basal body temperature used for?

BBT can be used for natural fertility tracking and as an indicator of pregnancy. BBT rises during ovulation and stays elevated throughout the remainder of the cycle until menstruation occurs. If a woman’s BBT does not return to the baseline temperature, it could indicate pregnancy.

Women's average baseline basal body temperature is 97.0- 98.0°F. An individual BBT measurement can be affected by a multitude of factors, including sickness, stress levels, inconsistent sleep cycles, alcohol consumption, travel through time zones, and certain medications. Because of this, BBT is highly individualized to each person, and it is best analyzed over the month to draw meaningful conclusions about a person’s health.

What wearables measure basal body temperature?

Fitness trackers provide a unique advantage if you want to analyze changes in your basal body temperature: a consistent and reliable temperature measurement. A consistent temperature measurement allows the slight elevation to be more easily detected. These wearables will measure and track your BBT:

• Fitbit: The Sense, Charge 4, and Charge 5 models take skin temperature readings throughout the night and will show your individual baseline and variation after just 3 nights.

• Oura: The Oura Ring 3 takes skin temperature readings and combines that with other data, such as heart rate variability and breathing rates, to predict both ovulation, menstruation, and even COVID-19 infection. Oura also partners with Natural Cycles, an FDA-approved app for cycle tracking.

• Whoop: The Whoop 4.0 band also measures skin temperature to help inform your daily readiness score, but it does not include specific features for ovulation or menstruation.

• Ava: This wearable is specifically designed to detect skin temperature and predict menstruation and ovulation. It measures skin temperature throughout the night and presents a fertility prediction in the morning through the app. Ava combines skin temperature data with heart rate variability, resting heart rate, breathing rate, and skin perfusion to make its prediction.

How accurate are wearables at measuring basal body temperature?

Measuring BBT in a consistent and uniform way can be challenging since different areas of the body will give different temperature readings. Utilizing a wearable to measure BBT, or skin temperature, can provide more precise measurements and allow you to draw more accurate and helpful conclusions. Some wearables will take hundreds, sometimes even thousands, of temperature measurements daily to provide an even more accurate picture of BBT. By taking measurements over the course of the day, a person can analyze how other factors influence their BBT. The knowledge about how different foods, work schedules, and stressors affect the physiological state of the body can help a person make more informed day-to-day decisions about what is best for their health.

A study was conducted on the effectiveness of the Oura Ring in predicting fertility cycles. The slight increase in distal body temperature, along with changes in heart rate variability, detected by the Oura Ring were effective in predicting fertility cycles for 100% of the participants in the study.

This is for informational purposes only and should not be considered medical advice. Go to the emergency room if you are experiencing a life-threatening medical emergency.

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Blog written by Allie Kantor and reviewed by Milos Tomovic.