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The Basal Body Temperature is the temperature of the body taken in the morning, after at least 8 hours of sleep and before any physical activity including getting out of bed, moving around in
bed, talking, eating or drinking. Any physical activity after waking, no matter how slight, will affect the Basal Body Temperature. The temperature should be taken with either a mercury or electronic
thermometer which displays increments of 0.05º C (0.1º F). This type of thermometer will detect the very small changes in body temperature associated with the menstrual cycle.
A woman's Basal
Body Temperature usually drops from between 12 to 24 hours before ovulation and then rises sharply over a period between 24 to 48 hours. This sharp rise (approximately between 0.2º to 0.5º C/0.4º F to
1.0º F) is called the 'thermal shift' and is due to high progesterone levels. The period of time when a woman is least fertile is considered to begin after 3 days of temperature elevation and to last until
the start of the next menstrual cycle or period.
The rate and pattern of the increase in Basal Body Temperature varies greatly from woman to woman and somewhat from cycle to cycle in any one woman.
Several cycles need to be observed and careful records kept in which the woman takes her temperature at the same time every morning before doing anything else. It may be taken orally or rectally but should
be done in the same way every day. After 6 months the temperature patterns are compared and the average time of ovulation can be determined. A woman is considered to be most fertile during the 6 days prior
to the earliest recorded date of ovulation from that 6 month period until the 5th day after the rise in temperature in the current cycle.
CONSIDERATIONS
This method is not an absolutely accurate
way of predicting when ovulation occurs and the most or least fertile times for a woman can be difficult to determine. There are many factors which can influence the Basal Body Temperature to give a
misleading reading such as; stress, infection, a bad night's sleep, medication, use of an electric blanket or extremes of temperature. Body temperature is also difficult to interpret just before and during
menopause. If a woman does not wish to become pregnant it is advisable to use one or more additional methods of contraception such as condoms and/or the contraceptive pill. |
