Basal Body Temperature (BBT)
The body's resting temperature, measured orally or vaginally first thing in the morning after at least three hours of uninterrupted sleep and before any activity, eating, or drinking. BBT rises 0.2 to 0.5 degrees C within one to three days after ovulation due to the thermogenic effect of progesterone released by the corpus luteum; this sustained shift produces a characteristic biphasic pattern on a cycle chart. A thermal shift confirms that ovulation has occurred but cannot predict ovulation in advance. BBT is therefore useful for retrospective ovulation confirmation, luteal phase length measurement, and detection of a short luteal phase (<10 days) or anovulation, but is not reliable as the sole marker for identifying the fertile window. Accuracy depends on consistent timing, adequate sleep, and the absence of confounders (illness, alcohol, disturbed sleep, shift work). BBT is a core component of sympto-thermal methods and a secondary confirmatory marker in several other FABMs.
This content is for educational purposes only and does not constitute medical advice. Consult an RRM clinician or healthcare provider for guidance specific to your situation.