BBT - or Basal Body Temperature Charting - allows you to predict ovulation based on charting changes in your body temperature throughout your menstrual cycle.

Basal Body Temperature and Ovulation Prediction

One of the many changes that take place in a woman's body during her menstrual cycle is an increase in body temperature at the onset of ovulation. During the first the first part of a woman's cycle, the body temperature is lower. With ovulation, the body temperature rises (to create a more fertile environment for the fertilized egg) and remains higher until the beginning of a woman's next cycle.

Recording each day's body temperature - and charting the patterns of temperature change - will help pinpoint when when ovulation has occurred.

What is basal body temperature? Basal body temperature (or BBT) is the temperature of the body at rest. The BBT method requires charting your basal - think 'base' - body temperature when you're completely at rest. For most women, 96 to 98 degrees is considered normal prior to ovulation and 97 to 99 degrees after ovulation. By charting the differences - in one-tenth degree increments - you can determine when ovulation has taken place.

How to take your Basal Body Temperature

Your Basal Body Temperature can be taken orally with a special BBT thermometer [1]. Ideally, a larger thermometer that registers from 96 to 100 degrees (in easy-to-read one-tenth degree increments) is recommended. Digital thermometers [1] are also available. Remember, you are attempting to discern as little as a two-tenths of a degree of difference in temperature, so selecting a quality thermometer and following directions are crucial.

Important: Your BBT temperature refers to a 'resting' or 'base' temperature. That means that your BBT must be measured prior to any physical activity, after at least three to four hours of sleep. Make sure that you read thermometer directions - and if you have any questions or unique considerations, consult your physician.

1. Taking Your BBT Temperature
Take your BBT temperature first thing each morning - as soon as you wake up. It is recommended that you remain in bed (as any physical activity can increase your resting temperature), as well as avoid eating or drinking. Coffee can wait! Either insert the thermometer in your mouth - or alternatively your rectum - and wait five minutes. Read the temperature to within 1/10 of a degree and record the reading.

2. Charting Your BBT Temperature
Starting on day one of your menstrual cycle - the first day of your period - begin recording your BBT temperature using a calendar or notebook and graph paper. Each morning, record your temperature. Plot each day's BBT on the graph. Your temperature rise may be sudden, gradual, or in steps. The pattern may vary from cycle to cycle.

3. Predicting Ovulation
For most women, 96 to 98 degrees is considered normal prior to ovulation and 97 to 99 degrees after ovulation. By charting the differences - in one-tenth degree increments - you can determine when ovulation has taken place. Typically a rise of at least 0.4 to 0.6 degrees will take place at ovulation, though for different women the temperature increases may be sudden or gradual.

In selecting a basal thermometer, we recommend using a specially designed digital thermometer [1] (that reads two digits after the decimal, e.g., 97.75) or a newer glass thermometer that does not contain mercury [2]. Mercury thermometers are being phased out, do to environmental issues, so if you select a glass bbt, make sure its a mercury-free basal thermometer.

Part II
Frequently Asked Questions and BBT Limitations [3]