Basal body temperature (BBT)
The normal human body oral temperature averages at about 98.6°F (37°c) but varies during
the day, especially in response to exercise. A woman's body temperature not only fluctuates
throughout the day, it also responds to hormonal changes throughout the month, thereby
giving a valuable information about individual fertility cycle. What is BBT?
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.
Changes in BBT
In an average 28‐day menstrual cycle, with ovulation typically occurring around day 14, the
BBT shows two phases. In the first phase, during the menstrual period and through to
ovulation, the BBT is at its lowest level. Soon after ovulation, however, the BBT rises
approximately 0.4°F (at least 0.2°c) and stays raised until menstruation starts; this is the
second or post‐ovulatory phase. The rise in temperature is due to the hormone progesterone,
which is secreted by the corpus luteum after the ovum has been released, and is a sign that
ovulation has already occurred.
Factors affecting BBT
As the post‐ovulatory rise in BBT is quite small, disturbances in temperature can be confusing.
The following are a few of the more common factors that could give a variation in your BBT.
Your body reaches its BBT after you have asleep for four hours or more, so if you work night
shifts or take naps you should take your BBT after your main sleep of the day. If you often get
disturbed in the night or early morning, but usually manage to go back to sleep for another
couple of hours, remember to record your BBT after your longest sleep. Take your
temperature before you get out of bed or have a hot or cold drink because your temperature
can change quite quickly.
If you have been drinking alcohol during the evening, your BBT may be high the next morning
so don't confuse this with your mid‐cycle rise, which should stay raised until your period.
Although a hot teddy late at night can help you get to sleep, alcohol will actually disturb your
sleep so that you sleep less deeply and may cause you to wake up a few hours later. And if
you've been up late the night before your BBT may be raised the next morning, which makes
for a confusing temperature reading the next day!
Stress, anxiety, illness and infection can raise your BBT temporarily, as will some drugs that
are used to treat menstrual disorders, such as progestogens.
Air travel and the resulting jet leg can affect your 24‐hour temperature cycle, so bear this in
mind when you interpret your overall pattern.
If you have repeated, unexplained spikes of high temperature, check that you are shaking
down the mercury in your thermometer properly each morning. If you have a sudden dip in
temperature it is more likely to be a recording.
How to take your BBT So that you can take your BBT accurately, you will need to purchase a special glass fertility
thermometer which has each degree marked in tenths. You can also use an unbreakable
electronic digital thermometer. This is a easier to use than a glass thermometer because it
makes a beeping sound when a stable temperature is reached. It also store the information
until you are ready to read or record it. If you are using a glass fertility thermometer
remember to shake it down well before using.
Take your temperature first thing in the morning before you move around under your arm,
hold for 5 minutes and read. Using a basal body temperature ( BBT) is an ultrasensitive thermometer that tracks your
body’s exact temperature. A digital thermometer is your best bet. These are fairly cheap and
can be purchased at any pharmacy.