Your body temperature shifts subtly during your menstrual cycle, rising after your ovulate. If you check your temperature each day, you can track when ovulation has occurred. Basal body temperatures are a simple way to keep track of your cycle, but they are limited, because you don’t know that you have ovulated until after the fact – and then, it’s too late to plan to have intercourse in order to get pregnant.
If you want to try tracking your cycles, take your temperature first thing in the morning before you get out of bed. Try not to move too much, as activity can raise your body temperature slightly. Use a glass basal thermometer or a digital thermometer so that you can get accuracy to the tenth of a degree. Keep the thermometer in your mouth for five minutes. If your temperature is between two marks, record the lower number.
Try to take your temperature at the same time every day if possible. Shake the thermometer down when you are done so that you do not have to shake it in the morning and thus risk raising your temperature from the movement.
After you ovulate, your body temperature will rise and stay elevated for the rest of your cycle. If you don’t get pregnant, it will fall at the end of your cycle, and you will get your period. Create a chart and write down your temperature everyday. From one day to the next, your temperature will zigzag a little. These small temperature changes will seem random at first - ignore them.
Also, ignore the occasional "fluke" temperature that is obviously way out of alignment with the others -- this can happen for any number reasons (like stress) and not important to finding pattern. If you look at a complete cycle, you will probably notice a point at which the temperatures become higher than they were in first part your cycle. More specifically, the rise is when your temperature increases 0.2 degrees above the previous six days. That temperature jump occurs just after ovulation.
The limitation with monitoring your temperature is that by the time you are certain that you have ovulated, it is usually too late! You can still try to get pregnant the morning your temperature rises, but chances are slimmer. The egg is probably gone by that point.
Nevertheless, temperature tracking can be helpful. After several cycles, you may be able to see a predictable pattern and get a sense for your most fertile days. The rise also lets you know when you are less likely to become pregnant if you have intercourse. And lastly, temperature is a good indicator of whether you are pregnant. If your temperature does not go down at the end of your cycle, you probably succeeded!