Family Planning: A Global Handbook for Providers, a 2012 WHO publication related to calendar-based contraceptive methods:
Basal body temperature – the body temperature of a fertile woman when resting – rises after ovulation and remains high until the next menstruation.
Detailed method: Every day after waking up, immediately (before getting up or eating any food) measure body temperature and record it in a special designed form; after ovulation, body temperature usually rises 0.2-0.5°C/0.36-0.9°F (this usually happens at the middle stage of the menstrual cycle).
Basal body temperature is the lowest body temperature attained during rest (usually during sleeping). It is generally measured immediately after awakening and before any physical activity has been undertaken.