Basal Body Temperature
Dr. Elizabeth Pryor answers the question, Can charting your BBT help you identify fertile times?
What is basal body temperature, and why does it matter when trying to conceive?
Basal body temperature (BBT) is your body's lowest temperature during the day. Your temperature will rise slightly at the time of ovulation, so charting your BBT is a relatively simple and inexpensive way of documenting your most fertile times. The difference in temperature will be slight (about 0.4 degrees), but it will stay elevated for the last half of the cycle.
To chart your BBT, you'll need a special thermometer (available at any drugstore for about $10). You'll need to take your temperature every morning at the same time, so keep your thermometer by the bed and use it as soon as you wake up. Once your temperature has remained elevated for three days, you have ovulated. The key is to chart your BBT for a least two months so you can see the pattern of your normal cycle.