Basal body temperature method
The basal body temperature (BBT) method is based upon the fact that a woman's temperature drops 12 to 24 hours before an egg is released from her ovary and then increases again once the egg has been released. Unfortunately, this temperature difference is not very large. It is less than 1 degree F (about a half degree C) when the body is at rest.
The basal body temperature method requires that a woman take her temperature every morning before she gets out of bed. A special thermometer that is more accurate and sensitive than a typical oral thermometer must be used, and the daily temperature variations carefully noted. This must be done every month. Online calculators are available to help a woman chart her basal body temperature.
To use the basal body temperature as a birth control method, a woman should refrain from having sexual intercourse from the time her temperature drops until at least 48 to72 hours after her temperature increases again.