A woman's basal body temperature during pregnancy will remain slightly elevated from the time of conception through about the end of the first trimester. In most cases, this small increase in resting temperature is not noticeable except with the use of a basal thermometer, which can accurately read body temperature to 0.1°F (0.18°C). Many women use the slight fluctuations in basal body temperature to determine when they are most fertile and then as an early indicator of pregnancy.
Though there are normal fluctuations in basal body temperature, during pregnancy it is generally between 0.4 and 1.0°F (0.072 and 0.18°C) higher than between a woman's period and the time of her ovulation. If a woman's basal body temperature remains elevated for more than 15 days or so after ovulation, this could be an early indication that she is pregnant. False negatives are not uncommon with pregnancy tests, but a stable, slightly increased temperature from the time of the most recent ovulation for two or more weeks is a strong indicator that the woman has conceived. During this time, pregnancy tests are often unreliable, so many women use this temperature to determine whether or not they might be pregnant.
The basal body temperature will remain at this slightly elevated level for the first trimester of a normal pregnancy. At the end of the first trimester, the woman's temperature returns to normal. A sharp increase is probably an indication that she is fighting off an infection or disease.
Many women use basal body temperature to help them determine their fertility cycles. The temperature is usually fairly consistent between the end of a woman's period and her next ovulation. As a woman ovulates, her basal body temperature will drop a few tenths of a degree and then rise sharply, between 0.4 and 1°F (0.072 and 0.18°C). The temperature will remain elevated until the woman begins her period, at which point the cycle starts over again. If a woman has conceived, her body temperature should remain elevated, so a decrease in this temperature is a strong indicator that she has not conceived during a given ovulation cycle.