| BBT or 'Basal Body Temperature' is a measure of your resting body temperature and should be taken first thing in the morning before you get out of bed. Charting your BBT helps to establish the phases of your menstrual cycle and enables you to work out exactly when ovulation occurred - it is the only charting method that enables you to do this, the others simply predict when it is likely to occur.
What you need
If you are planning on using BBT to monitor your fertility and identify your peak fertility times you will need a special BBT thermometer; digital and mercury varieties are available. Regular fever thermometers are not suitable as they are only likely to display temperature to 0.2deg rather than the necessary 0.1deg accuracy.
The thermometer should be kept on your bedside table so you can take your temperature first thing on waking before you get up or have a drink or even speak (this is so a true reading of your resting temperature can be taken). You will also require a menstrual cycle chart to plot your daily temperature on - you should record the time your temperature was taken as well as any other features monitored on this chart
How it works
BBT charting works on the basis that your post-ovulation temperature (during the luteal phase) is at least 0.4deg higher than your temperature prior to ovulation (in the follicular phase). Average pre-ovulation BBTs typically range between 97deg - 97.7deg and 97.7deg - 99.0deg post-ovulation, although this varies between individuals.
Charting your BBT not only enables you to establish when ovulation has occurred but also the typical duration of your menstrual phases. Although the average cycle length is 29 days with ovulation on day 14, this is not the case for many women, especially those with irregular periods. Variation in the menstrual cycle is likely to be due to the follicular phase, by contrast, the luteal phase is likely to remain constant each cycle. BBT enables you to establish the length of the luteal phase and therefore work out post-ovulation fertility and when to test for pregnancy.
You tend to be most fertile 5 days before, and 2 days after you ovulate, so charting for several months will help you to establish when in your cycle this period falls. 18 days of increased temperature is a good indicator of pregnancy and additionally, this method enables you to work out the date of conception and due date relatively accurately.
The temperature increase seen at ovulation is due to an increase in progesterone cause by the corpus luetum (the follicule from which the egg was released). If fertilisation of the egg occurs, BBT will remain high throughout the first trimester. However if fertilisation doesn't occur, the corpus leutem disintegrates and progesterone levels drop causing temperatures to lower and menstruation to occur.
To get the most reliable measurements you should take your BBT on waking each morning. It is best if you take your temperature at the same time each day, even on the weekends as BBT increases by approximately 0.1deg every half an hour and so could skew the results. Although it is possible to adjust the temperature by adding or subtracting 0.1deg for every half an hour earlier or later BBT is recorded compare to the usual time, this isn't advisable on a regular basis and you should make a note of the adjustment on your chart.
It is best to take your temperature after at least 3 hours of continuous sleep and to get a more reliable result at least 5 hours is preferable. It is possible to take BBT orally or vaginally and although orally measuring BBT is easiest first thing in the morning, some believe that vaginally provides a more reliable result as it avoids the influence of factors such as sleeping with your mouth open etc. It is important to use the same method (and thermometer) each day. It is also advisable to mark on your chart days when your temperature may be unreliable due to illness or alcohol consumption etc.
Monitoring your BBT
You should begin to take readings on day 1 of your menstrual cycle (this is the first day of your period) and continue for each day until the first day of your next period when you should begin a new chart.
After a complete cycle you should join the temperatures for each day (in the form of a line graph)and look for a jump in temperature. When you see the BBT shift you should draw a line 0.1deg above the 6 previous highest readings - this is known as a cover line - and your temperature should stay above this line until 1 or 2 days before the end of the cycle.
Although BBT is useful for establishing when ovulation has occurred, it is best to use it in conjunction with other fertility charting measures such as cervical mucus and cervical position. By combining different charting methods you will be able to establish when you are most fertile and predict the period in which you are most likely to conceive. | 
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