What is basal body temperature (BBT)? Your BBT is the temperature you take when your body is at a complete rest. Throughout the month your BBT will change depending on a number of factors, including your hormones. It is the lowest measurement of the body’s temperature and used as a baseline. Day one of your cycle, your temperature should drop with a period and remains lower in the first half of your cycle. Just before you ovulate, your temperature will drop slightly and then