BBT: How Does It Change Throughout a Cycle?

Please explain basal body temperature changes as they relate to menstruation, ovulation and conception?


A "normal" basal body temperature (BBT) chart is "biphasic," meaning there are two temperature phases. The early temperatures before ovulation are lower than temperatures during the next half of the cycle, following ovulation.

The average BBT is 97.48F plus or minus 0.25 (36.37 ± 0.12C) during the phase prior to ovulation (follicular phase) and 98.09F plus or minus 0.22 (36.72 ± 0.12C) in the phase following ovulation (luteal phase). If conception occurs, the temperature stays at the elevated level and never goes down.

A shift in BBT, following ovulation, to the "hyperthermia phase" of the cycle should occur within a period of 48 hours or less. If you are charting your temperatures, you will typically see that three consecutive daily BBTs are at least 0.36 degrees F (0.2 Celsius) higher than the previous six daily temperatures.

This BBT rise precedes or coincides with ovulation and is associated with the increased production of progesterone by the ovary. Your BBT chart does not predict the day of ovulation, but rather provides evidence of ovulation two or three days after it has occurred. This can be helpful in predicting ovulation once several months have been charted and you are able to see a pattern.

A biphasic BBT usually indicates that ovulation has taken place, although a monophasic (no change in temperature throughout cycle) BBT may be observed in some ovulatory cycles.

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