The basal body temperature method - a fertility awareness-based method - is a type of natural family planning. Your basal body temperature is your temperature when you're fully at rest. Ovulation may cause a slight increase in basal body temperature.
You'll be most fertile during the two to three days before your temperature rises. By tracking your basal body temperature each day, you may be able to predict when you'll ovulate. In turn, this may help you determine when you're most likely to conceive.
If you're hoping to get pregnant, you can use the basal body temperature method to determine the best days to have sex. Similarly, if you're hoping to avoid pregnancy, you can use the basal body temperature method to help determine which days to avoid unprotected sex. Because the basal body temperature method alone doesn't provide enough warning time to effectively prevent pregnancy, it's generally used in combination with other fertility awareness-based methods if you're hoping to avoid pregnancy.