What is Basal Body Temperature? BBT is your body's lowest resting temperature, measured immediately after waking, before getting up, eating, drinking, or even talking. It reflects your underlying hormonal state, not external activity.

Why track it? After ovulation, the hormone progesterone causes a sustained rise of 0.2–0.4°C (0.4–0.7°F). Charting this thermal shift confirms ovulation has occurred and marks the start of your infertile post-ovulation phase. Used alongside cervical mucus observations, BBT gives you a powerful two-sign picture of your fertility.

What thermometer to use? Use a dedicated BBT thermometer that reads to two decimal places (e.g. 36.52°C / 97.74°F); a standard fever thermometer isn't sensitive enough. Oral measurement is most common; vaginal or rectal are also valid. Pick one location and stay consistent throughout your cycle. Take it at the same time each morning after at least 3–4 hours of uninterrupted sleep.

Reading the chart Pre-ovulation temps are typically lower (36.1–36.6°C / 97.0–97.9°F). Post-ovulation temps are higher and stay elevated (36.4–37.0°C / 97.5–98.6°F). The amber dashed line on the chart is your coverline, drawn 0.2°C (0.4°F) above the average of your lowest pre-ovulation readings. When 3 consecutive temperatures sit above the coverline, ovulation is confirmed. A drop back to pre-ovulation levels near the end of your cycle signals your period is coming within 48 hours.