Basal body temperature, or BBT, is your body's temperature at complete rest, and it is most accurate when measured as soon as you wake up, before getting out of bed. After ovulation the hormone progesterone rises, and this raises your temperature by about 0.2 to 0.5 degrees Celsius, roughly 0.4 to 1 degree Fahrenheit. The rise usually lasts until your next period. Because the difference is small, the chart drawn over many days means more than any single reading.
The key feature of BBT tracking is that it shows ovulation has happened after the fact. When your temperature stays above the previous days for several days in a row, ovulation most likely happened a day or two earlier. This information comes too late for that cycle, but logged over several cycles it shows which day ovulation usually falls on in your cycle. Used together with ovulation tests, the picture becomes clearer.
For reliable readings, measure at the same time every day after at least three hours of uninterrupted sleep. Illness and fever, alcohol, poor sleep, measuring late or shift work can affect the values. Noting these days helps you tell misleading rises apart when reading the chart. A rise that lasts longer than 18 days can be a sign of pregnancy and is a reason to consider taking a pregnancy test.
What you can do
- Log your morning reading with date and time, to 0.01 degree precision.
- Flag factors that affect the result, such as illness, alcohol, poor sleep or a late reading.
- See the last 30 days on a chart, with ovulation marked once a rise is detected.
- Summarizes your average, lowest and highest values, number of readings and post-rise average.
- Shows a note suggesting a pregnancy test if the rise lasts longer than 18 days.
- Supports Celsius and Fahrenheit; get an AI pattern analysis once you have at least 5 entries.
How to use it
- Take your temperature at the same time each morning, as soon as you wake and before getting out of bed.
- Tap 'Add Temperature', enter the value and flag any factor that affected the reading.
- Watch the chart for your temperature to rise above the previous days and stay there.
- Keep going for several cycles to see which days ovulation falls on.
Frequently asked questions
How do you take your basal body temperature?
Basal body temperature is measured as soon as you wake up, before getting out of bed, talking or drinking anything. Measure at the same time every day and after at least three hours of uninterrupted sleep. Use a sensitive digital thermometer, ideally one that shows two decimal places, and always measure the same way, for example by mouth. Logging the value right away keeps you from forgetting it.
How much does your temperature rise after ovulation?
After ovulation, progesterone usually raises basal body temperature by 0.2 to 0.5 degrees Celsius, roughly 0.4 to 1 degree Fahrenheit. Because the rise is small, instead of looking at a single reading you check whether it stays above the previous days for at least three days in a row. The rise usually lasts until your period starts, when the temperature drops again.
Does basal body temperature drop in early pregnancy?
In pregnancy progesterone stays high, so basal temperature usually stays elevated. A rise lasting longer than 18 days can be a sign of pregnancy. A one-day dip means nothing on its own; measuring time, sleep and room temperature also affect the value. If you think you may be pregnant, the most reliable step is to take a pregnancy test and review the result with your doctor.
Can BBT predict ovulation?
No, basal temperature does not predict ovulation in advance; it shows it after it has happened. By the time you see the rise, the most fertile days have most likely passed. Still, charts from several cycles reveal which day ovulation usually falls on and help you plan future cycles. For advance notice, ovulation tests and tracking cervical mucus are better suited.