The temperature method (basal body temperature method) is based on the knowledge that a woman's body temperature is subject to hormone-related fluctuations during the menstrual cycle.

During the first half of the cycle (the phase of follicle maturation) the body temperature is usually below 37°C. About two days after ovulation the temperature increases by 0.2 to 1°C. It remains raised during the second half of the cycle (the luteal phase) and does not return to the initial value below 37°C until menstruation begins.

The increase in temperature is triggered by the corpus luteum which produces the hormone progesterone after ovulation. Progesterone has a direct influence on the temperature center in the brain.

Daily measurement of the basal body temperature (BBT) is used to determine the time of ovulation. The basal body temperature is the body temperature which is measured in the rectum immediately before getting up in the morning. Ideally it should be measured at the same time every day. A digital thermometer accurate to two places after the decimal point should be used.

By carefully charting her basal body temperature a woman can identify her fertile and infertile days. The WHO recommends the following procedure: 'A genuine temperature rise can be recognized by the fact that three consecutive temperatures are at least 0.2°C (0.4°F) higher than those measured on the previous six days. The first day of the elevated temperature is defined as the temperature rise. At the time of the temperature rise fertilization is unlikely to occur. At this time it is 2 days since ovulation and the ovum has already died.'

The basal body temperature chart can also be used to detect a pregnancy or identify abnormalities of the menstrual cycle.

The temperature method is only moderately reliable at most. Getting up later, sleeping fewer hours, physical exercise or a mild cold are sufficient to disrupt the measurements. There are also some women who do not have a marked progesterone-induced temperature increase.

The temperature method can be combined with regular observation of the cervical mucus (cervical mucus method). This is called the symptothermal method and is considerably more reliable.