Your lowest resting body temperature, measured on waking to help detect ovulation.
Why it matters for PCOS
Basal body temperature is your lowest resting temperature, usually taken with a basal thermometer first thing in the morning before getting out of bed. After ovulation, the hormone progesterone raises the baseline slightly (typically around 0.3 degrees Celsius), so a sustained temperature shift across a few days of charting can show that ovulation has occurred.
For women with PCOS, BBT can be harder to interpret because ovulation is often irregular or unpredictable, so a chart may not show a clear pattern. It confirms ovulation after the fact rather than predicting fertile days in advance, so it is most useful alongside other methods and a conversation with your doctor.
This glossary is education, not diagnosis. For your own situation, ask a doctor or pharmacist.