In natural family planning, a woman's basal body temperature can be used as an indicator of fertility. When you combine the basal body temperature method with the calendar method, you can better predict fertile days and lessen the risk of unintended pregnancy.
How basal body temperature works
A woman’s temperature rises slightly (between 0.4 and 0.8 degrees) when she ovulates. If you take your temperature every morning before you get out of bed using a special high-resolution thermometer, and if you discover a rise one day (and you’re sure that you don’t have an infection of some sort to account for the rise), then you can presume that you have ovulated and should refrain from unprotected sex.
Advantages of the basal body temperature method
Because this is really an accompaniment to the calendar method, the same advantages apply: it does not require any doctor appointments and it comes without side effects.
Mostly, however, basal body temperature is best used as proof that the calendar method is working. Having some measure of proof of natural family planning (aside from not being pregnant) can provide reassurance.
If you want to get pregnant, the basal body temperature is a good predictor of when you should be having intercourse to make a baby.
Disadvantages of the basal body temperature method
The same disadvantages of the calendar method apply as well: it is not a particularly reliable method of birth control, especially for women with irregular cycles. Plus, outside factors, such as a lack of sleep, can cause a woman’s temperature to vary.
Remember that you cannot use basal body temperature alone because sperm can live for two to seven days and any sperm deposited ahead of time can still impregnate the woman. This means that you must also follow the calendar method, which precludes sex during much of the month.
Even if you are presumably safe from becoming pregnant, that doesn’t mean that you are safe from getting a sexually transmitted disease. Unless you are certain that your partner is 100 percent safe, you should make sure that a condom is in place before attempting intercourse.