Basal Body Temperature Method
Also known as the sympto-thermal method, this birth control method involves charting your core body temperature throughout your menstrual cycle. Since your temperature rises slightly around the time of ovulation, finding the sustained spike in your core temperature will alert you as to when you are most fertile during your monthly cycle.
For this natural birth control to work, a woman needs to take her temperature every day, first thing in the morning before she gets out of bed. And by first thing, that means you can't run to the washroom quickly or put coffee on first. Your temperature needs to be taken before you even kick off those sheets.
Once you have your temperature, record it on a chart so that you can follow the fluctuations in your temperature from day to day. Generally, your temperature should hover around 97.2ºF and 97.4ºF, although it may go up or down from day to day. When you ovulate, though, your temperature will increase anywhere from half a degree to as much as a full degree (often going above 98ºF) and will remain elevated due to the extra production of progesterone.
Seeing a sustained increase in your temperature, one that lasts for at least three days, indicates that ovulation has taken place. When you know ovulation has taken place, it can be assumed that having sex during the rest of your menstrual cycle will not result in pregnancy. Before you ovulate, though, it can be a bit harder to determine exactly when the "safest" time is to have sex.
Once the ovaries have released an egg, there is a 24-hour window during which time the egg can be fertilized. However, sperm can remain viable inside a woman for as long as seven days. Therefore, it is possible to become pregnant even if you don't have sex during your most fertile time. For this reason, some couples using this method choose to abstain from sex prior to ovulation. Known as the post-ovulatory temperature method, with perfect use it can be as much as 99% effective.
Some couples prefer not to abstain from sex for such a long period and instead examine the temperature patterns of a woman's previous cycles in order to determine about when ovulation occurs. The "safest" time to have sex is thought to be up to one week before the earliest recorded day of temperature rise.
The symptom-thermal method requires the use of a special type of thermometer that is designed to read the minute changes in your temperature and only has a range of a few degrees. These can be purchased at your local pharmacy. While basal temperature thermometers are available in the traditional glass thermometer style, many women prefer the new digital thermometers, as they are easy to use and clearly display your temperature. Basal temperature thermometers are relatively inexpensive, ranging in price from $10 to $15.
How you record your temperature is up to you. Many women find that it is easiest to use a chart which allows them to see all of their temperature fluctuations during the given cycle. Using a chart also allows you to record other information that can offer you more insight as to what part of your cycle you are in, such as cervical mucus, breast tenderness and mood swings.
On average, the basal body temperature method has a failure rate of 15% but as low as 2% per year for perfect users. To help make your readings more accurate, and therefore make your charting more reliable, you should try to take your temperature at about the same time everyday. If you are sick during the month or forget to take your temperature, your charting for that month may be inaccurate.