Basal body temperature (BBT) is your body's lowest resting temperature, measured immediately after waking before any activity. It's one of the most informative and completely free tools available for tracking your cycle — used for both helping you conceive and for understanding your hormonal health.
Why BBT Matters for Fertility
After ovulation, progesterone causes your basal body temperature to rise by about 0.2–0.5°C (0.4–1.0°F) and stay elevated until your period arrives. This temperature shift is the only reliable at-home confirmation that ovulation has occurred.
Important caveat: BBT confirms ovulation after it has happened — it doesn't predict it. For predicting your fertile window in advance, you need to combine BBT with cervical mucus observation or use ovulation predictor kits (OPKs).
How to Take Your BBT Correctly
- Use a basal thermometer (reads to 2 decimal places — standard thermometers aren't precise enough)
- Take your temperature at the same time every morning, before getting up, talking, or drinking
- You need at least 3 consecutive hours of sleep before taking your temperature
- Oral, vaginal, or rectal — pick one method and stick to it throughout your chart
- Record it immediately — don't rely on memory
A temperature taken an hour later or after a restless night will be skewed. Note these disturbances on your chart so you can disregard those data points rather than misread your pattern.
Reading Your BBT Chart
A typical BBT chart shows two phases:
- Pre-ovulation (follicular phase): Lower temperatures, typically 36.2–36.5°C (97.0–97.7°F)
- Post-ovulation (luteal phase): Higher temperatures, typically 36.5–37.0°C (97.6–98.6°F)
The shift between these two phases — called the thermal shift — marks ovulation. To confirm ovulation, you want to see your temperature rise at least 0.2°C above the previous 6 days and stay elevated for at least 3 days.
Pre-ovulation dip
Many women see a slight temperature dip the day before ovulation as oestrogen peaks. This can be a useful advance signal.
Thermal shift
Temperature rises 0.2–0.5°C after ovulation and stays elevated through the luteal phase. This confirms ovulation occurred.
Triphasic pattern
A second temperature rise after the initial post-ovulation shift — around 7–10 DPO — may indicate implantation and pregnancy.
Temperature drop
If your temperature drops back below the coverline, your period is likely coming in 1–2 days.
What BBT Can Tell You
After 2–3 charted cycles, you can identify:
- Whether you're ovulating — a monophasic flat chart with no thermal shift may indicate anovulation
- When in your cycle you ovulate — not necessarily day 14, especially with irregular cycles
- Your luteal phase length — should be 10–16 days; shorter may indicate low progesterone
- Possible pregnancy — temperature staying elevated past 18 DPO is a strong sign
- Thyroid issues — consistently very low or very high baseline temperatures can prompt further investigation
BBT Charting and PCOS or Irregular Cycles
BBT charting is particularly useful if you have irregular cycles or PCOS, because it doesn't assume you ovulate on day 14. It shows you when — or whether — ovulation is actually happening. If you have PCOS, you may see multiple potential shifts that don't resolve clearly, or long stretches of low temperatures. Sharing this data with your doctor gives them meaningful, cycle-specific information.
Limitations of BBT Charting
BBT confirms ovulation but doesn't predict it. Your most fertile days are the 2–5 days before ovulation, and by the time your temperature rises, that window has closed. This is why BBT works best in combination with:
- Cervical mucus monitoring — egg-white cervical mucus signals your approaching fertile window
- Ovulation predictor kits (OPKs) — detect the LH surge 24–36 hours before ovulation
Used together, these three methods (the "Fertility Awareness Method" or FAM) give you both advance warning of your fertile window and confirmation that ovulation occurred.
Track Every Step of Your TTC Journey
Two Pink Lines is a printable planner designed for women trying to conceive — track cycles, ovulation signs, symptoms, appointments, and emotional milestones in one beautiful tracker.