The basal body temperature (BBT) is a person's at-rest temperature.

Natural cyclic changes in female hormones usually cause a woman's basal body temperature to go down slightly just before an egg is released (ovulation) and then go up sharply 24 hours after ovulation. By carefully measuring BBT every morning before getting out of bed and recording it on a chart, many women are able to estimate when they are ovulating. This helps pinpoint when a woman is most and least likely to become pregnant.

The change in your body temperature is very slight, so you need to use a special thermometer to measure it. You can use a regular digital thermometer, or you can 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.

Keeping track of basal body temperature may be helpful for women trying to get pregnant or trying to avoid pregnancy. It is one of several fertility awareness or natural family planning methods of birth control.

• Having sex during the 5 days before and the day of ovulation increases your chances of becoming pregnant.
• Avoiding sex up to and until several days after you have ovulated may help you prevent pregnancy, because the human egg is typically fertile for only 12 to 24 hours after ovulation.

Basal body temperature charting is done at home and is inexpensive. But to get an accurate record, a woman needs to track her temperature every day for several months. Women who work varying shifts or who have irregular menstrual cycles may have difficulty getting a useful basal body temperature measure.