Human body temperature to give the most accurate assessment of core body temperature, particularly in hypothermia. In the early 2000s, ingestible thermistors in capsule form were produced, allowing the temperature inside the digestive tract to be transmitted to an external receiver; one study found that these were comparable in accuracy to rectal temperature measurement. In the 19th century, most books quoted "blood heat" as 98 °F, until a study published the mean (but not the variance) of a large sample as . Subsequently that mean was widely quoted as "37 °C or 98.4 °F" until editors realised 37 °C is closer to 98.6
Basal body temperature typically lasts for at least three days and can survive as long as a week, making prediction of ovulation several days in advance necessary for avoiding pregnancy. There is limited evidence about the effectiveness of fertility awareness family planning methods, some of which use basal body temperature as one component. About 24% of women who use any type of fertility awareness program become pregnant during the first year, compared to about 85% of sexually active women who are not trying to prevent a pregnancy. The World Health Organization ranked fertility awareness methods, taken as a whole, as an "effective" method