Basal body temperature

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 decrease slightly just before an egg is released (ovulation) and then increase 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 thermometer used for measuring the BBT is marked in tenths of a degree, making it possible to detect even small rises in temperature. When the temperature goes up—usually about 0.4ºF (0.2ºC) to 1.0ºF (0.6ºC)—and stays up for several days in a row, ovulation has occurred.

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. However, 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 chart.