Basal Body Temperature (BBT)
Definition
The lowest body temperature attained during rest, traditionally measured immediately upon waking before any activity.
Why It Matters
BBT is primarily used in fertility awareness: - Ovulation detection - Temperature rises 0.2-0.5°C after ovulation - Cycle tracking - Confirms ovulation occurred (retrospectively) - Fertility planning - Helps identify fertile window - Cycle irregularities - Reveals patterns or issues - Natural family planning - Part of symptothermal method
How It's Measured
Traditional BBT measurement: 1. Use a basal body thermometer (more precise than regular) 2. Measure immediately upon waking 3. Same time each day 4. Before getting up, eating, or drinking 5. Record to 0.1°C precision
Apple Watch alternative: Wrist temperature provides similar insights with less user effort.
Typical Pattern
| Cycle Phase | Temperature Pattern |
|---|---|
| Follicular (pre-ovulation) | Lower, relatively stable |
| Ovulation | May dip slightly |
| Luteal (post-ovulation) | Rises 0.2-0.5°C, stays elevated |
| Menstruation | Drops back to baseline |
Confounders & Limitations
- Timing - Must measure at consistent time
- Sleep quality - Poor sleep affects readings
- Illness - Fever disrupts pattern
- Alcohol - Evening drinking affects morning temperature
- Travel - Time zone changes disrupt pattern
- Measurement consistency - Different thermometers vary
- Retrospective only - Confirms ovulation after it happens
Use Cases
- Fertility awareness - Natural conception planning
- Cycle monitoring - Understanding your cycle
- Hormone health - Detecting anovulatory cycles
- Natural family planning - Part of combined methods
Practical Advice
- Consistency is key—same time, same method
- Track for several cycles to see your pattern
- Combine with other fertility signs for accuracy
- Don't rely solely on BBT for contraception