Basal Body Temperature (BBT)

Basal body temperature (BBT) is the body’s lowest resting temperature, typically measured first thing in the morning, before any activity, such as sitting up, talking, or getting out of bed.


🧬 Why BBT matters:

BBT is influenced by hormonal changes during the menstrual cycle — especially the rise of progesterone after ovulation. Tracking BBT is one of the key tools in fertility awareness methods for:

  • Identifying ovulation
  • Understanding the fertile window
  • Confirming the luteal phase length
  • Detecting possible pregnancy

🔁 How BBT changes across the menstrual cycle:

Cycle PhaseHormonal ActivityBBT Pattern
Follicular PhaseEstrogen dominantBBT remains relatively low and steady
OvulationLH surge and egg releaseSlight dip in some women, then a rise
Luteal PhaseProgesterone rises post-ovulationBBT increases by 0.4–1.0°F (0.2–0.6°C)
Pre-MenstruationProgesterone drops (if no pregnancy)BBT drops back down to follicular levels
PregnancyProgesterone remains highBBT stays elevated past 16+ days

📏 Typical BBT range:

  • Pre-ovulation (follicular phase): ~97.0–97.7°F (36.1–36.5°C)
  • Post-ovulation (luteal phase): ~97.8–98.6°F (36.6–37.0°C)

🛠️ How to measure BBT:

  1. Use a BBT thermometer (more sensitive than a standard thermometer)
  2. Take temperature every morning at the same time after at least 3–4 hours of sleep
  3. Record immediately — even a small rise matters
  4. Track over multiple cycles to identify patterns

Summary

TermDefinition
Basal Body TemperatureThe body’s lowest resting temperature, best measured upon waking.
PurposeUsed to detect ovulation and identify fertile or infertile phases of the cycle.
Indicator of ovulationA sustained temperature rise of ~0.4°F or more, lasting at least 3 days.