Final answer:
Basal body temperature is the lowest body temperature when the body is at rest, usually during sleep. It rises after ovulation occurs and can be used to determine when ovulation has occurred. However, it cannot predict in advance when ovulation will occur.
Step-by-step explanation:
Basal body temperature is the lowest body temperature when the body is at rest (usually during sleep). It is most often estimated by a temperature measurement taken immediately upon awakening in the morning and before any physical activity has occurred. Basal body temperature normally rises after ovulation occurs, as shown in Figure 22.11.5. The increase in temperature is small but consistent and may be used to determine when ovulation occurs, around which time unprotected intercourse should be avoided to prevent pregnancy. However, basal body temperature only shows when ovulation has already occurred, and it cannot predict in advance when ovulation will occur. Sperm can live for up to a week in the female reproductive tract, so determining the occurrence of ovulation only after the fact is a major drawback of this method.