What Can Basal Body Temperature Tell You About Your Cycle?

Basal body temperature (BBT) is your temperature at complete rest, taken first thing on waking. After ovulation, the hormone progesterone raises it by a few tenths of a degree, and it stays higher until your next period. Charted over a cycle, that step shows that you probably ovulated and roughly when, but only after it’s happened: BBT confirms ovulation rather than predicting it.

This article is general information, not medical advice.

Why the Temperature Rises

In the first half of the cycle, before ovulation, temperature is lower. After ovulation, the empty follicle produces progesterone, which acts on the brain’s temperature control and raises your resting temperature. The rise is small, a few tenths of a degree. In a study by Maijala and colleagues, women’s oral temperatures averaged 0.23 °C higher in the luteal phase than in the follicular phase.

A review of ovulation detection methods by Su and colleagues describes BBT as cheap and simple, but notes its limits: the rise comes after ovulation, so it can’t identify the fertile days ahead of time, and it’s easily disturbed. It’s also not always clear-cut: some ovulatory cycles show no obvious shift.

Taking a Reading You Can Compare

1. Take it immediately on waking, before sitting up, talking, or drinking anything.
2. Take it at about the same time each day.
3. Use the same thermometer at the same site, ideally a digital basal thermometer that reads to 0.01 °C.
4. Note anything that could raise it, such as illness, alcohol, a late night, or a different time.

Reading the Chart

Look for a shift: several days in a row clearly higher than the days before, lasting until your next period. Ovulation has usually happened shortly before the shift. Because some ovulatory cycles show no clear shift, a flat chart doesn’t by itself mean you didn’t ovulate.

BBT and Wrist Temperature

Watches and rings measure skin temperature overnight, as a change from your own baseline, which avoids the morning routine. It shows the same luteal rise; see What Does Your Sleeping Wrist Temperature Tell You? The two measure different things, so they shouldn’t be mixed on one chart.

Following It in Kaizora

Basal Body Temperature is on the Vitals and Menstrual Cycle pages, logged by hand or imported from Apple Health, kept apart from other temperatures. Menstrual Cycle charts your cycles aligned by cycle day, with flow, tests, and temperature on each, so the luteal step lines up from cycle to cycle. Kaizora doesn’t predict ovulation or fertile days from it; see Why Are Period and Fertile-Window Predictions So Often Wrong?

The Short Version

• BBT rises a few tenths of a degree after ovulation and stays up until your next period.
• The rise confirms ovulation afterwards; it can’t predict it.
• Measure on waking, before getting up, the same way every day.

Sources

• Su HW, Yi YC, Wei TY, Chang TC, Cheng CM, “Detection of ovulation, a review of currently available methods,” Bioengineering & Translational Medicine 2, no. 3 (2017): 238–246. doi:10.1002/btm2.10058
• Maijala A et al., “Nocturnal finger skin temperature in menstrual cycle tracking: ambulatory pilot study using a wearable Oura ring,” BMC Women’s Health 19 (2019): 150. doi:10.1186/s12905-019-0844-9