What temperature measures
After ovulation the ruptured follicle becomes the corpus luteum and secretes progesterone. That hormone is thermogenic: it lifts baseline body temperature by 0.2 to 0.5 °C. The shift is small but clear on a chart, and it holds until the day before the next period.
It is worth being exact about what this sign can do. A temperature rise confirms that ovulation has happened. It says nothing about the day before, still less about the four days before that — during which sperm may have been deposited and waited. The thermal shift always arrives too late to protect the opening of the fertile window, and that is a structural limit, not a technique failure.
Taking the reading
- On waking, before getting up, before drinking, talking at length or moving about.
- After at least three hours of unbroken sleep. Below that the reading means nothing.
- Always at the same site: oral, vaginal or rectal. The values are not interchangeable; switching mid-cycle makes the chart unreadable.
- With a two-decimal thermometer, kept from cycle to cycle.
- At a roughly stable hour. An hour's difference is enough to move the value.
- Noting disturbances on the day: short night, alcohol the evening before, fever, cold, waking in the night, travel, medication.
The three-day rule
Ovulation is taken as confirmed once three consecutive readings exceed the previous six, the third being at least 0.2 °C above the highest of those six. The infertile phase then begins on the evening of the third day and runs to the next period.
Clearly disturbed values may be discounted — provided this was noted on the day, not explained away afterwards because the chart is inconvenient.
Effectiveness
Used alone and with intercourse confined to the post-ovulatory phase, the method is very safe: the fertile window is avoided entirely. But it imposes abstinence over half the cycle, and few couples sustain that.
Used alone but with intercourse early in the cycle, it becomes far less reliable, since nothing in it signals ovulation approaching. That practice is what produces the high failure rates sometimes attributed to the method.
In practice, then, temperature is a component rather than a complete method: combined with mucus observation it forms the symptothermal method, whose effectiveness is of an entirely different order.
What a chart teaches beyond contraception
Three or four cycles of readings give useful information well past the contraceptive question: whether ovulation occurs at all, whether the luteal phase is of normal length (12 to 16 days), whether cycles are regular. It is a common reason to start charting — including when trying to conceive, or before a fertility consultation.
Do not rely on temperature alone
Fever, a broken night, jet lag, alcohol, shift work, a small child who wakes: the chart blurs quickly. A cycle can also be anovulatory with no advance warning. Temperature gives no protection against sexually transmitted infections.