When you ovulate, your body's core temperature rises by a half to one degrees. Since this change is so slight, few women even know it occurs. However, charting when this temperature rise occurs in you can help you know just when you are most fertile every month.

Changes You Expect During Your Cycle
Basal body temperature, or BBT, is lower during the first two weeks of the menstrual cycle, prior to ovulation. The presence of the hormone estrogen keeps the BBT low. Typically it will range from 97.0 to 97.5 degrees Fahrenheit (F). Immediately following ovulation, progesterone "turns up the heat" a bit, and there is typically a rise of at least 0.4 to 0.6 degrees until the time of your next menstrual period. This temperature rise will let you know that ovulation has occurred. If your BBT remains elevated even past the time that your menstrual period is due, it could be a very early indicator of pregnancy.

Measuring Your Basal Body Temperature
When beginning to take your basal body temperature, it may be helpful to follow these guidelines:

When measuring your BBT, it is important to use a special thermometer intended for this purpose.
Shake down your thermometer before you go to bed --if using a mercury thermometer.
Keep your thermometer in easy reach, next to your bed.
Use a charting method to keep track of your cycle .
Take your temperature first thing in the morning -- even before getting up to go to the bathroom. It is important to take your temperature as close to the same time every morning as possible.
Keep your thermometer in place for five minutes before reading.
Presence of a fever, emotional upset, insufficient sleep, consumption of alcohol or the use of an electric blanket or heating pad can affect your BBT.