Final answer:
During ovulation, a woman's basal body temperature slightly rises due to an increase in progesterone levels after ovulation.
Step-by-step explanation:
The nurse explains that during ovulation, the woman's basal body temperature (BBT) will rise slightly. This is a natural family planning method used to monitor the menstrual cycle to prevent conception. After ovulation occurs, the increase in progesterone levels results in a small but consistent rise in BBT, which may be used to determine the occurrence of ovulation. To prevent pregnancy, unprotected intercourse should be avoided during the fertile window identified by tracking BBT, though it is important to note that BBT only shows when ovulation has already occurred, not in advance.