About Basal body temperature (BBT) BBT is the lowest body temperature attained during rest. It is usually estimated by a temperature measurement immediately after awakening and before any physical activity has been undertaken. Ovulation causes an increase of 0.5 to 1 degree Fahrenheit in BBT. So monitoring BBT helps to estimate the day of ovulationt BBT ThermometerAn accurate thermometer is required to measure tiny BBT changes. With the accuracy of 0.09 degree Fahrenheit, Femometer basal thermometer is a professional female fertility thermometermeter Features1. Femometer measures oral temperature, which is more accurate than armpit temperature and more convenient than rectal temperature.2. Data is uploaded to the complimentary app via Bluetooth. The intelligent app auto generates BBT curve which eliminates manually entering results.3. Femometer has no screen. Three beeps mean BBT is taken, then you can absolutely fall back to sleep. During the whole process, you don't even need to open your eyes.4. The device can store up to 300 BBT data, which allows you to sync with the app later. All data is securely stored in the Cloud. 5. In the calendar of the app, it auto marks your fertile window, period and ovulation date. You are able to record daily notes to improve the accuracy of prediction.6. The app allows you to set up alarm clock. You can easily measure BBT at the same time every day, which helps to get more accurate prediction.7. Knowing your fertility requires taking BBT on a daily basis. The app will provide a more comprehensive understanding after 3-month usagention1. It may take up to three minutes to measure BBT at first. The more you use it, the less measuring time it will take. The average measurement time is 90 secs.2. The battery usually lasts for 6 months. Battery model is CR1632 button cell for future replacement.