What is the BBT Charting Tool?

Looking for an explanation of our charting tool? Here it is! For step by step directions, read below and also click here. The Pregnancy.org Base Basal Temperature (BBT) Charting Tool is designed to help women pinpoint their ovulation as an aid in determining ovulation as a method of natural birth control.

All BBT charting tools rely on a combination of ovulation-induced temperature shift, combined with other physiological signs, to pinpoint ovulation.

The Pregnancy.org Charting Tool differs from other charting tools by relying on the core math used to determine ovulation, rather than shortcuts created to reduce charting complexity when charting without the aid of computers. Despite that the Pregnancy.org Charting Tool uses sophisticated mathematical models internally these are largely transparent to the chart user: the computers that run the application do the hard work.


To pinpoint the temperature shift most tools track temperature then look for a temperature spike. The temperature shift is supposed to be 2/10ths of a degree above the running average of the prior five days.

Popular books on the subject list a variety of patterns for the shift, but ultimately recommend a "you'll know it when you see it" approach." Most medical studies of BBT temperature as an ovulation predictor use a different approach. Studies rely on traditional statistics, and especially on a statistical measure known as "standard deviation."

Standard deviation

: measures how far a number is from the center of a collection of other numbers -- a type of mega-average. Each measure is known as a deviation from the mean.

The Pregnancy.org BBT Charting Tool looks at the standard deviation from the beginning of a menstrual cycle to the present days measurement and determines how many standard deviations the current measurement is from the running average.

Early in a cycle number fluctuate wildly so a higher number of standard deviations should be used to "see" a temperature spike, which predicts ovulation. Later in the cycle there is more data -- more days -- so a smaller number of standard deviations can be relied upon as predicative of ovulation.

Since every women's physiology is different both the number of standard deviations and the "window" -- the number of days -- those deviations measure are customizable. Customization can be either on a month-to-month basis or set for all months. To change the number of standard deviations, or the number of days they look at, choose the Customization link from the respective charting screen.

The following link has some charts the delve deeper into the concept of standard deviation, explaining how to compute it using Microsoft Excel and illustrating the concept with daily temperature samples. This isn't the same technology the Pregnancy.org BBT Charting tool uses, but it is the same underlying mathematical principles. Note the Charting Tool uses the equivalent of the STDEVP function (discussed towards the bottom of the article).

Non-Temperature Based Predictors

There are several documented predictors besides temperature that help pinpoint ovulation. These are cervical position and firmness, cervical fluid (also known as cervical mucus), vaginal sensation, and/or the use of a fertility kit or ovulation predictor kit. These predictors are well documented elsewhere.

Depending again upon physiology and personal belief women may choose to use one or more of these indicators, ignoring or lessening the significance of others. The Pregnancy.org BBT tool supports this by allowing women to indicate which predictors to use when trying to pinpoint ovulation. Each predictor is given a "weight" -- from 0 to 100 -- that the tool then takes into account when trying to determine ovulation. A weight of zero means the predictor is tracked by the tool, but not used when trying to pinpoint ovulation. A weight of 100 means that individual predictor is the only predictor used.

For example, assigning cervical mucus a weight of 100, and temperature a weight of 0, would tell the tool to track temperature but ignore it when predicting ovulation. A weighing of 50 each would tell the tool to determine if ovulation looked likely on a given day based on an equal weighing of temperature spikes and cervical mucus.

To encourage the more accurate counts the mucus indicator used to pinpoint ovulation is customizable. It may be changed from egg white to "sticky" or "creamy" depending again upon personal preference.

There is no right or wrong weighing of individual predictors: these are different for every woman and may even vary from month to month. Additionally, the most accurate physiological method may not work well due to personality issues. For example, a "space cadet" who frequently forgets to take her temperature first thing in the morning may wish to rely more heavily upon an alternate indicator, despite that temperature may be more accurate if taken consistently.


While the Pregnancy.org BBT Charting tool uses a more accurate calculation engine, it relies upon the same conventions used by other charting application. For example, skipping a day of temperature input plots a dashed line indicating the day was skipped and the value not used in computing ovulation. Similarly, marking that a day is supposed to be ignored -- because temperature was taken at the wrong time, or for several other reasons -- plots the say with an outlined point and ignores it in internal calculations.

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