Basal body temperature

Basal body temperature (BBT) is the body’s temperature at complete rest, usually taken orally, vaginally, or rectally immediately upon waking and before any activity. It changes subtly in relation to hormonal shifts during the menstrual cycle: it tends to be lower in the first half of the cycle and rises slightly—often by 0.4 to 1.0 degree Fahrenheit—after ovulation due to increased progesterone.

To detect ovulation, a sustained temperature shift—typically lasting at least three days—is considered a reliable sign that ovulation has occurred. Charting BBT over multiple cycles can reveal patterns in cycle length, luteal phase duration, and potential anovulatory cycles. While useful, it only confirms ovulation retroactively and must be tracked daily at the same time for accuracy.

Many people use digital thermometers and apps to track BBT trends. Temperature can be affected by illness, alcohol, poor sleep, or waking at inconsistent times, so maintaining consistent measurement habits is important. Over time, individual patterns emerge, helping users understand their unique cycle rhythm and anticipate fertility windows.