Basal body temperature: what its rise shows about ovulation

Basal body temperature (BBT) is measured on waking, before any activity. The NHS guidance on natural family planning (2024) lists it as one of three signs recorded in fertility awareness, with vaginal discharge and the dates of the cycle. It differs from the other two in one important way: it changes after ovulation rather than before it.

Basal body temperature rises after ovulation and stays higher until the next period

The American College of Obstetricians and Gynecologists (ACOG), in its patient guidance on fertility awareness-based methods, describes a rise of 0.5 to 1°F around ovulation, about 0.3 to 0.6°C, that lasts until the end of the cycle, through the luteal phase (ACOG, 2019). The rise follows the release of an egg. A chart from a cycle with ovulation therefore shows two levels, lower before ovulation and higher after it, a pattern clinicians call biphasic. A cycle without ovulation usually stays at one level (American Society for Reproductive Medicine, ASRM, committee opinion on fertility evaluation, 2021).

A temperature chart confirms ovulation after it happens and cannot predict it

By the time a rise shows on the chart, ovulation has passed, and so has the fertile window for that cycle, which ends on the day of ovulation (Wilcox, Weinberg and Baird, New England Journal of Medicine, 1995). ACOG (2019) notes that the method identifies ovulation only after it has occurred, and places the most fertile days in the two to three days before the rise. For timing sex in the current cycle, the chart comes too late. Its value is the record it builds: over several cycles it can show whether ovulation is happening and roughly when it tends to fall. The guide to ovulation symptoms shows which signs come before ovulation and which after.

Charting takes one reading each morning, before getting out of bed

ACOG (2019) describes the method: the temperature is taken every morning after waking, before any activity, before getting out of bed and before eating or drinking, and it is recorded each day. A fever, whether from an illness such as influenza or from an ongoing medical condition, makes the readings unreliable. The rise is small, so the chart is read as a pattern across several days rather than from any single reading.

Basal body temperature is less precise than urine LH tests

In a study of 101 women attending for infertility, the temperature chart agreed with ultrasound evidence of ovulation in 74 in 100 cycles, compared with 98 of 101 cycles for urine LH tests, and the lowest point on the chart, sometimes read as the day of ovulation, predicted it poorly (Guermandi and colleagues, Obstetrics & Gynecology, 2001). ASRM's 2021 committee opinion on fertility evaluation calls temperature charting inexpensive but often unreliable, notes that some women who do ovulate never show a clear two-level pattern, and does not routinely recommend it as a test.

An older idea holds that the fertile days continue after ovulation. The authors of a 2000 study of the fertile window point out that the evidence for fertile days after ovulation came from studies that used crude measures of ovulation, basal body temperature among them. With more precise hormone measures, the fertile window does not seem to extend beyond the day of ovulation (Wilcox, Dunson and Baird, BMJ, 2000).

Temperature is most useful alongside a sign that changes before ovulation

Cervical mucus and the urine tests in ovulation predictor kits change before ovulation, so they can mark the fertile days in the current cycle. Recording mucus and temperature together is called the symptothermal method (ACOG, 2019). ASRM's 2022 committee opinion on optimizing natural fertility cites a prospective study of 2,832 cycles in which mucus changes tracked the temperature record closely and predicted peak fertility more accurately than a calendar.

ACOG (2019) is clear that temperature on its own is not a good way either to prevent or to achieve pregnancy. The NHS (2024), whose guidance on fertility awareness is written for avoiding pregnancy, says the method takes 2 to 3 cycles to learn and that it needs to be taught by an expert such as a fertility awareness practitioner.