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 Phase | Hormonal Activity | BBT Pattern |
---|---|---|
Follicular Phase | Estrogen dominant | BBT remains relatively low and steady |
Ovulation | LH surge and egg release | Slight dip in some women, then a rise |
Luteal Phase | Progesterone rises post-ovulation | BBT increases by 0.4–1.0°F (0.2–0.6°C) |
Pre-Menstruation | Progesterone drops (if no pregnancy) | BBT drops back down to follicular levels |
Pregnancy | Progesterone remains high | BBT 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:
- Use a BBT thermometer (more sensitive than a standard thermometer)
- Take temperature every morning at the same time after at least 3–4 hours of sleep
- Record immediately — even a small rise matters
- Track over multiple cycles to identify patterns
✅ Summary
Term | Definition |
---|---|
Basal Body Temperature | The body’s lowest resting temperature, best measured upon waking. |
Purpose | Used to detect ovulation and identify fertile or infertile phases of the cycle. |
Indicator of ovulation | A sustained temperature rise of ~0.4°F or more, lasting at least 3 days. |