Basal body temperature (BBT) is the temperature measured at rest, usually immediately after waking. After ovulation, progesterone often raises the temperature compared with earlier days. This pattern can help confirm retrospectively that ovulation probably happened, but it cannot predict the ovulation day on its own.

What a basal body temperature chart shows

The useful signal is a change over several days: lower temperatures before ovulation followed by a sustained rise. One reading, a fixed numeric range, or an app cannot confirm ovulation by itself. Fever, alcohol, interrupted sleep, shift work, travel, stress, and some medicines can also change a reading.

BBT is therefore best understood as a trend record. It mainly answers “has ovulation probably happened?” rather than “will ovulation happen tomorrow?”. This matters when trying to conceive: by the time the rise is visible, the most fertile part of the cycle may already have passed.

How to measure consistently

  • Use the same thermometer and, when possible, the same measurement route.
  • Measure on waking, before getting up, talking, eating, or drinking.
  • Keep the time and sleep routine reasonably consistent without treating small changes as a diagnosis.
  • Record fever, short sleep, alcohol, travel, shift work, and other factors that may explain an outlier.
  • Track more than one cycle before drawing conclusions about your personal pattern.

BBT and the fertile window

Because the temperature rise usually appears after ovulation, BBT alone has limited value for anticipating fertile days. For planning intercourse, combine the chart with signs that occur before ovulation, such as cervical mucus changes, or with a detection method recommended by a clinician. The app can show a calendar estimate and, in trying-to-conceive mode, use sufficient temperature data to confirm probable ovulation and calculate DPO. This information is not a clinical assessment.

A regular cycle also does not guarantee the same ovulation day every month. When trying to conceive, intercourse every one to two days during the fertile window generally covers cycle variation better than waiting for the temperature rise.

What BBT cannot diagnose

A chart cannot diagnose luteal phase deficiency, low progesterone, infertility, polycystic ovary syndrome, or thyroid disease. Luteal phase length varies, and the ASRM notes that there is no simple, reliable clinical test for diagnosing luteal phase deficiency in a natural cycle. Repeated patterns or concerns about ovulation should be discussed with a gynecologist or fertility specialist.

Using the data safely

Bring charts and notes to an appointment if you cannot identify a pattern across several cycles, your cycles are very irregular, you have unexpected bleeding, or pregnancy has not occurred within the timeframe recommended for your age and medical history. Tracking supports the conversation; it does not turn normal variation into disease.

How Levvi helps you track

Levvi lets you record basal temperature, cervical mucus, symptoms, and events that may affect a reading. For example, if interrupted sleep comes with one unusually high reading, record both and compare the following days before interpreting a trend. The app estimates ovulation from cycle history; in trying-to-conceive mode, it can use sufficient temperature data to indicate probable ovulation and calculate DPO. Use these insights to organize observations and conversations with your healthcare team.