Using a Basal Thermometer
This method relies on precise recording of your body's resting temperature. For this method to be accurate, temps must be taken first thing in the morning, and at the same time each morning. The temperature must be recorded before ANY activity occurs, activity will raise the body's temperature and cause a misleading temperature reading. What I do is set my alarm next to my bed within arm's reach, when it goes off in the morning, I roll over, turn it off, and put my digital thermometer in my mouth, take my temperature, when that is done, then I can get up to go to the bathroom, or get a drink of water, or go back to sleep.
There is a really great book that covers this method in great detail, written by Toni Weschler, I highly recommend anyone considering fertility awareness methods, even if you choose not to use the Sympto-Thermal method, to get this book. Its called Taking Charge of Your Fertility - The Definitive Guide to Natural Birth Control and Pregnancy Achievement. I wish I had this book when I was learning fertility awareness methods, even though I opted not to use a basal thermometer (using a plastic speculum instead) to confirm the passage of fertility each cycle, still it would have given me lots of valuable information during those learning years.
This method is fairly involved, the actual taking and recording the temperatures on a chart is actually pretty easy, but when temperatures don't show the "text book" fertility patterns, it can be difficult to interpret and confusing for women who are learning these methods, Ms Weschler goes into a lot of detail explaining how to interpert the BBT charts, I haven't had a lot of experience with this method, so I'll cover the basics to help get you started, but I strongly recommend getting Toni's book.
What is a basal thermometer? A basal thermometer is different from a regular fever thermometer which measures temperatures in degrees. A basal thermometer measures temperatures by 10th's of a degree. A digital thermometer will also work.
After ovulation occurs the hole left behind by the egg is transformed by luteinizing hormone (which also caused ovulation to occur) into what is called the corpus luteum, which means "little yellow body". Its function is to secrete progesterone, when the endometrium is stimulated by progesterone it begins to grow and thicken in preparation in case an egg is fertilized this cycle. As progesterone gets into the bloodstream it causes the body's resting temperature to increase noticeable. Pre-ovulatory temperatures are usually below 97.4 degrees. Once ovulation occurs temperatures will rise a day or two later, and will stay higher than pre-ovulatory temperatures until menstruation begins. Usual post-ovulatory temperatures are at or above 97.7 degrees.
In order for this method to help you avoid pregnancy:
When recording temperatures, you'll need a chart to record them on to aid in helping you to see the fertility patterns. I've found several online. Just do a search for "BBT Chart" and you'll come up with a few. The clearer your records/charts are the more useful they will be to you. When you use a chart (or graph) to record your temps, when you draw lines to connect the dots (your daily temps) it will show your fertility pattern, and if ovulation has occurred, the sustained temperature rise will be easy to identify.
When learning the sympto-thermal method, its a good idea to take your temperature every day for a minimum of 6 cycles. During this time, contraception should be used every time as well. It takes time to get to know your cycle, and get the hang of identifying your mucus patterns, discovering what is normal for you. Before choosing not to use contraception, users of fertility awareness methods need to be positive that ovulation has occurred.
Remember: Your temperature does not tell you when you will ovulate. It only tells you when it is done.
You will have relatively low temperatures from the time of your period until you ovulate. Ovulation will cause a rise of about 6/10th of one degree: .6 F., or six lines on your graph. This rise can happen in a day or it can stair-step up over a period of several days.
So to avoid pregnancy, avoid having unprotected sex until the your temperatures are recorded for three consecutive days at .4 F higher than your temperatures were for the six days previous to the rise.
Take a reading after at least 3 hours of sleep, before any activity, before getting out of bed, eating, drinking, or smoking, because these will cause the BBT to rise, giving a false high temperature reading.
Try to take your temperature within an hour and a half of the same time each morning upon awakening. If you sleep extra late, your temperature may make a false rise, so be sure to mark your calendar if you sleep late. Set your alarm clock for earlier than you would normally wake up, take your temperature, then go back to sleep. This method is best suited to those who have a morning routine.
Temperatures taken vaginally or rectally are more accurate than a temperature taken orally. Use only one over the course of a month because there is about a degree difference between oral and vaginal or rectal temperatures. Though oral temperatures are just fine, and probably more convenient. I've always taken it orally.
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