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.
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 also 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
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.
A basal body thermometer (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. Each thermometer kit comes with a blank graph. These
graphs may have space for you to record PMS symptoms.
You can record you temperature from the mouth, vagina, or rectum but be
consistent: your BBT will be slightly different depending on which route you
choose. If you use a glass thermometer leave it in place for five minutes before
reading the temperature. If the mercury falls between two numbers record the
lowest temperature every time. A digital thermometer will need about a minute to
"beep" with a temperature reading.
BBT and Ovulation Prediction
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. 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.
Guidelines for an Accurate BBT Chart
The first day of your menstrual flow is day 1 of your BBT chart. Do NOT
include spotting prior to your period as day 1. Your temperature should drop
when your menstrual flow starts. Record your temperature throughout your
period. This information is important.
Make sure you note the actual day of the month in the space provided on
Use an oral, digital, basal body thermometer only. A regular thermometer
Take your temperature each morning before you get out of bed. Place the
thermometer under your tongue for at least 2-3 minutes.
Don't eat, drink, or smoke before you take your temperature (this
includes ingesting sperm!).
Record your temperature by using a dot, not an X or a checkmark.
Use a down-pointing arrow to indicate the days you had intercourse.
Record any premenstrual symptoms if there is space on the chart.
Otherwise, use your symptom chart to help link a certain temperature to a
Note any special considerations like true illness or fever.
Change charts when you get your period again.
Finally, on one of your charts, record what you're eating, how much you're
exercising, and whether you're under any unusual stress. Coffee, alcohol,
dieting and exercise, and emotional stress all affect your menstrual cycle. By
cutting out certain foods or vices, you can see if your menstrual cycle changes.
How long should a woman chart before seeing a doctor if one suspects
If your cycles are irregular, you shouldn’t waste time on BBTs alone - see a
doctor and find out what may be causing the irregularity. If you do have
normal-length cycles and decide to start charting, you only need to wait about 3
months to establish a problem and seek help. For example, if you have a 28-day
cycle, but ovulate on day 18, and that happens 2-3 months in a row, you should
see your doctor. Otherwise it depends on your age and urgency. It’s not a bad
idea for everyone to get preconception advice and bloodwork -- test for
immunities to rubella, chicken box, fifth disease, also test for
thyroid function at a minimum.
Dated 29 November 2011