Perhaps the best known of fertility prediction methods, and certainly the easiest to discuss in polite company, is the daily measurement of basal body temperature. Though doctors say it may be less reliable than observation of cervical mucus, it is recommended that the two methods be used together for maximum effectiveness.
Prior to ovulation, a woman’s basal body temperature is generally lower. Day to day temperatures vary, but remain in the comparatively low range. After ovulation occurs, a woman’s temperature rises suddenly, or “spikes.” Thereafter, temperatures remain high until the start of the next cycle. To get pregnant, sex should occur immediately before the temperature spike is observed, at the most fertile time. (Unlike the cervical mucus method, the basal body temperature method cannot predict impending fertility. The spike in temperature is seen after ovulation has already occurred, so if you wait to have intercourse until you see your temperature rise, you may be too late. However, if you use this method to chart several cycles, you may discover that they are regular, and that you can predict how many days into your cycle you usually ovulate.)
The basal body temperature method requires a bit more effort and quite a few more rules, but once you understand it, it should take less than five minutes of your time per day. In addition, you will need to obtain or create a chart for recording your daily temperatures. These charts can be found free by doing an online search, or they can be photocopied from a book. Temperatures on the graph generally run from about 97 degrees to 99 degrees, with a mark for every tenth of a degree in between. When taking and recording your temperature, there are a set of guidelines you should follow:
Take your temperature first thing each morning, before getting out of bed or doing anything else. If you are using a mercury thermometer, shake it down the night before and place it on your nightstand. (Shaking down the thermometer in the morning can elevate your body temperature.)
Use a special thermometer for basal body temperature. These can be found at any drugstore. A thermometer made to detect fever may not be sensitive enough.
Take your temperature either orally or vaginally, but make sure to use the same route each time. Do not use ear thermometers, as they are thought to be less accurate.
Try to take your temperature at the same time each day. If you sleep in, your temperature will tend to rise about a tenth of a degree for each half hour you sleep past your normal waking time.
Take your temperature after at least three hours of sleep. If you get out of bed an hour before your alarm goes off in the morning to use the bathroom, your temperature will be falsely elevated. It is better to record your temperature a bit early, if need be, than to check it after getting up.
Make a dot or a circle for your temperature each day, and connect the dots with a line, so you can see patterns.
If you think one day’s temperature might be falsely high or low because of difference in waking time or illness, you should chart it, but do not connect it to the rest of your line. Make a note to yourself about the possible reason for the discrepancy.
If your temperature falls between two numbers, record the lower of the two.
Observe your chart for the classic pattern: a series of lower temperatures, followed by a temperature spike (ovulation), and then a series of high temperatures. If your graph matches this pattern, you can be fairly certain that your most fertile day was on or slightly before the day of the temperature spike.