Basal body temperature rises after ovulation (in the luteal phase) and usually stays elevated for about 12 to 14 days as progesterone is released by the corpus luteum.
If the sperm has not met the egg, then the progesterone levels fall, temperature drops back down on the BBT, and menses ensue.
If however, pregnancy has occurred, progesterone will still continue to be released by the corpus luteum, and the temperature will remain persistently elevated and will not drop.
Patients have brought in their BBT charts with an unusual persistence of temperature elevation - this is almost always followed by a positive urine pregnancy test.