Situation: At the family planning clinic of a district hospital, the nurse counsels women and couples who want to space their children or have completed their families. A 30-year-old woman with regular cycles of about 28 days uses the basal body temperature (BBT) method. She brings her chart. Cycle day | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 Temperature (°C) | 36.3 | 36.2 | 36.3 | 36.1 | 36.6 | 36.7 | 36.7 | 36.8 Which interpretation of the chart is correct?

After ovulation, progesterone from the corpus luteum raises the basal temperature by about 0.3 to 0.5 °C, and the rise is sustained until the next period. Her temperature rises from day 14 and stays up, so ovulation most likely occurred around day 13 to 14. The fertile time ends only after the higher temperature has held for 3 days, not at the dip.

Interpretation of the BBT chart

The basal body temperature method rests on a clear hormonal sequence. Before ovulation, estrogen keeps the basal temperature relatively low. After the dominant follicle ruptures, the corpus luteum begins secreting progesterone, and progesterone acts on the hypothalamic thermoregulatory center to raise the resting temperature. The expected shift is approximately 0.3–0.5 °C, and the elevation should persist for the remainder of the luteal phase rather than appearing as a single isolated spike ^[1].

In the presented chart, the readings from cycle days 10 through 13 remain in a low, relatively flat range of 36.1–36.3 °C. Beginning on day 14, the temperature rises to 36.6 °C and then stays at 36.7–36.8 °C through day 17. This pattern—a sustained elevation of about 0.3–0.5 °C above the earlier follicular-phase baseline—is the classic biphasic pattern that indicates ovulation has most likely occurred around day 13 to 14. The temperature rise is a progesterone effect, not an estrogen effect, so it appears after ovulation rather than before it.

Watch out! The small dip on day 13 is not the end of the fertile window. Many charts show a transient preovulatory dip caused by a late estrogen surge, but the fertile period extends until the thermal shift has been confirmed. The fertile time is considered closed only after the higher temperature has been maintained for three consecutive days. On this chart, that confirmation is reached by day 17, not at the day 13 dip.

The size of the rise also matters less than its persistence. A shift of 0.3 °C or more that is sustained for at least 3 days is generally accepted as evidence of a thermal shift ^[3]. Here the rise from 36.1 °C to 36.6–36.8 °C exceeds that threshold and remains elevated, so the chart does not suggest anovulation. However, BBT interpretation is not perfectly reliable. In one evaluation, physician consensus on the ovulation day matched the luteinizing hormone peak within ±1 day in only 22.1% of ovulatory cycles, and an additional 22.1% were misread as monophasic ^[1]. Temperature fluctuation and cycle-length variability can also make concurrent interpretation difficult .

Chart feature	Meaning	Clinical implication
Low range days 10–13 (36.1–36.3 °C)	Follicular phase under estrogen influence	Fertile window may still be open
Dip on day 13	Possible preovulatory estrogen surge	Does not mark the end of fertility
Sustained rise from day 14 (36.6–36.8 °C)	Progesterone effect from corpus luteum	Ovulation likely occurred around day 13–14
Elevation held through day 17	Three-day confirmation of thermal shift	Fertile time can now be considered closed

Key point! BBT is a retrospective tool. It can suggest that ovulation has occurred, but it cannot reliably predict ovulation in advance, which is why couples using this method for contraception must abstain or use another method until the sustained rise is confirmed ^[1].