What is basal body temperature? Basal body temperature refers to the body temperature measured under the armpit or mouth after waking up in the morning when sleeping, eating, exercising, etc. remain unchanged. Generally, a woman\’s basal body temperature changes during the menstrual cycle. It rises before ovulation, remains high after ovulation, continues to rise if conception occurs, and falls before menstruation if pregnancy does not occur. How to measure basal body temperature? 1.Measurement time: