BBT Calculator | Basal Body Temperature

Following ovulation, your basal body temperature experiences a sustained upward shift, typically rising by about 0.5 to 1.0 degree Fahrenheit. This thermal shift is triggered by the release of progesterone from the corpus luteum, which warms your resting metabolic baseline. If pregnancy does not occur, progesterone levels drop right before your next period, causing your BBT to fall back down to pre-ovulation baseline levels.

In a pregnant cycle, your basal body temperature remains elevated past the standard length of a luteal phase. Because a fertilized egg implants and triggers continued high production of progesterone to support the uterine lining, your BBT stays high instead of dropping. A sustained high temperature lasting 18 days or more past ovulation is often one of the earliest physiological indicators of pregnancy.

In a non-pregnant cycle, your BBT stays elevated for roughly 12 to 16 days (the luteal phase) before dropping sharply right before or during menstruation. In contrast, a pregnancy chart displays a prolonged thermal shift known as a triphasic pattern or simply a sustained high luteal phase that does not dip. The elevated temperatures continue straight through the first trimester of pregnancy.

You can adjust your basal body temperature mathematically when you wake up at an irregular hour by using standard correction formulas. Generally, for every 30 minutes you wake up earlier than your baseline time, you add a fraction of a degree (such as 0.05) to your reading. Conversely, if you sleep in past your normal time, you subtract a small amount to account for the natural temperature rise that occurs with extra rest.