Your Basal Body Temperature

September 07, 2008

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Your Basal Body Temperature

by Lisa Rodriguez, R.N. and Marjorie Greenfield, M.D.
reviewed by Marjorie Greenfield, M.D.

Perhaps you have been in the baby-making mode for a while without success, or maybe you are just trying to better understand your cycles. An easy, inexpensive way to learn what is happening with your body's hormonal changes is to measure your basal body temperature, commonly referred to as the BBT.

What is basal body temperature?
A woman's temperature fluctuates during the day and during her menstrual cycle. Your lowest temperature each day is in the morning. Basal body temperature charting allows you to track your first morning temperature across the month. Before ovulation, your morning temperature is generally under 98 degrees Fahrenheit. After ovulation, it is usually over 98. Sometimes there is a dip in temperature on the day of ovulation.

Basal body temperature and fertility
Once you chart your temperatures for a month or two, a pattern begins to emerge and you can predict when you might be ovulating--thus, the best time to have intercourse and become pregnant. (Ovulation detection kits may be more accurate than the BBT in predicting ovulation, but are quite a bit more expensive.)

How do I take my BBT?

Basal body temperature thermometers can be purchased at your local drugstore for about $15. Regular oral thermometers are too hard to read since the BBT needs to be measured to the tenth of a degree. A digital thermometer also will work well.

Take your temperature orally with your BBT thermometer first thing in the morning. Since temperature fluctuates over each 24 hours, try to take it about the same time each day. To make this easier for you, have the thermometer at your bedside (and shaken down if it is the mercury type) the night before. To decide how long to take your temperature, follow the instructions that come with the thermometer. For best accuracy, you shouldn't drink, talk on the phone, go to the bathroom, or do anything else until you are finished taking your temperature each day.

After you take your temperature, be sure to write it down right away. Keep your BBT chart or a list of temperatures at your bedside. Don't rely on your memory--it is amazing how quickly the morning's number can leave your mind. (Nor can you make up for any memory lapse--taking your temperature later in the day will not accurately reflect your BBT.)

How do I graph my basal body temperature?

By convention, we start a new graph on day one of each menstrual cycle (i.e., the day you get your period). If you are starting later in the month, set up the chart with blank days for each day of your cycle that has already passed.

A BBT chart will be found in the instructions that come with your BBT thermometer, or you can obtain one from your practitioner or download one here from our website. You may want to make copies of the blank form for future use since you will make a new chart each month.

Keep in mind that many things can affect your temperature--infection, how much fluid you drink during the day, environmental temperature, etc. Particularly if you are sick, your readings may not be accurate. Make a notation on the chart if you are ill. It may also be helpful to note any medications taken and when you had intercourse throughout the month.

If you forget to take your temperature in the morning, leave that day blank. Taking your temperature later in the day will not be accurate.

After you have graphed your temperatures, connect the dots with a line. This will enable you to see when you have a change in temperature. During the time of ovulation, there is often a dip in temperature. After ovulation, hormonal changes should prompt a rise in your temperature that will last almost until your period begins.

Interpreting the BBT graph
Your most fertile time of the month is a few days prior to and during ovulation, which is just before your temperature rises. After graphing your BBTs for a three- to six-month period, you should have a much better idea about when you ovulate each cycle. Try not to overinterpret subtle findings on your BBT graph. How fast your temperature rises and exactly how high it goes does not correlate with how fertile you are. The BBT simply gives you general information about which days of the month are best to have intercourse to boost your chances of getting pregnant.

If you have been trying to conceive for six to twelve months without success, a visit to your practitioner can address any potential limitations and help speed up the process.


RELATED INFORMATION

			The Events of Conception
			The Events of Ovulation
			Trying to Conceive: Myths and Truths
			Getting Pregnant
			Infertility

Created April 05, 2001
Reviewed August 27, 2004

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