Your lowest resting body temperature, measured on waking to help detect ovulation.
What BBT is and how it links to ovulation
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, so a sustained temperature shift across a few days of charting can show that ovulation has occurred. It is worth remembering that BBT confirms ovulation after the fact rather than predicting fertile days in advance.
Why it is harder to read in PCOS
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 shift. Some cycles may pass without ovulation at all, leaving the temperature pattern flat across the month, while others may be so long that it is hard to tell when ovulation should have happened. For that reason, BBT is most useful when combined with other methods and a conversation with your doctor, rather than relied on as the only guide. See the what is PCOS page for broader context.
How charting works
For a meaningful chart, the temperature is taken with a basal thermometer at roughly the same time each morning, before sitting up or moving, because movement alone can change the reading. Factors such as disturbed sleep, fever, or travel can affect the pattern, so a single reading does not mean much on its own. What you are looking for is a sustained shift, not day-to-day ups and downs. Read more about other monitoring methods on the PCOS and fertility page.
Limits of this tool and when to see a doctor
Because BBT only confirms ovulation after it has happened, it is less useful for predicting fertile days in advance, and this is more noticeable when cycles are irregular, as they often are in PCOS. Some women find an unpredictable chart adds stress, while others feel it helps them understand their own body. If you are trying to conceive and your cycles are hard to read, a doctor can suggest other methods such as ovulation tests or monitoring, and explain the options for your situation.
This glossary is education, not diagnosis. For your own situation, ask a doctor or pharmacist.