Overview
What is basal body temperature tracking?
Basal body temperature (BBT) is your at-rest temperature. It's the lowest temperature a healthy person has during the day.
During your menstrual cycle, your changing hormone levels cause your BBT to fall 1 to 2 days before you ovulate. BBT then rises a day or two after you ovulate.
By carefully measuring BBT every morning before you get out of bed and recording it on a chart for a few months, you may be able to estimate when you ovulate. This helps pinpoint when you are most and least likely to get pregnant.
The change in your body temperature is very slight, so you need to use a special thermometer. You can use a regular digital thermometer or buy a basal thermometer. A basal thermometer shows you the temperature in tenths of a degree. This allows you to note tiny changes in body heat. Many wearable devices, such as activity trackers, can measure temperature. Some of these are approved by the U.S Food and Drug Administration (FDA) for tracking ovulation.
You can find basal or digital thermometers at a drugstore or online. You can also find kits that include materials for measuring and charting BBT.
When are you most likely to get pregnant?
About 2 weeks before your period, you will ovulate, which means that one of your ovaries will release an egg. You are most likely to get pregnant on the day of ovulation and the 5 days before it.
Your egg is fertile for about 12 to 24 hours after you ovulate. Avoiding sex until several days after ovulation may help you prevent pregnancy. Keep in mind that your cycle can change, making it hard to know when you are ovulating. Some people use this information as a form of birth control. But it isn't very reliable for preventing pregnancy.
How do you track your BBT?
To chart your basal body temperature (BBT), you can use an ovulation tracking app, preprinted charts, or graph paper.
To make your own chart using graph paper:
- Write a row of numbers at the top of your graph paper. Leave the first square in the row blank, then write the numbers 1 through 45, one number in each square. These are the days of your menstrual cycle. The first day of your period is Day 1. Your cycle may be shorter than 45 days. If it is, you don't need as many numbers.
- Write a vertical column of numbers that represent your temperature in tenths of a degree, on any given day. Start this column under the empty square next to the "day 1" square.
- If your thermometer is Fahrenheit: In the first square of your temperature column, write 99.0°. In the square under that, write 98.9°, then 98.8°, 98.7°, 98.6°, and so on, down to 97.0°.
- If your thermometer is Celsius: In the first square of your temperature column, write 37.5°. In the square under that, write 37.4°, then 37.3°, 37.2°, 37.1°, and so on, down to 36.0°.
Follow these steps to chart your BBT.
- Take your temperature at the same time every day before getting out of bed.
The change in your basal body temperature is very slight, so you need to use a digital thermometer or a basal thermometer.
- Record your temperature each day for several months.
If you're using a printed chart or graph paper:
- Shade the box on the grid that matches your temperature for each day.
- Draw a line connecting the shaded squares so you can see your dips and spikes.
If you have a somewhat regular cycle, the chart will help you predict when you will ovulate next. Just before your ovary releases an egg, your BBT dips slightly. Then, 24 hours after the egg's release, your temperature rises sharply and stays up for several days.