what is basal body temperature?
basal body temperature, often shortened to bbt, is your body's resting temperature, taken first thing in the morning before you get up, eat, drink or even sit up in bed. because it is measured under such consistent conditions, small shifts in bbt across the cycle can offer a useful window into hormonal changes, particularly around ovulation.
why bbt changes across the cycle
before ovulation, bbt tends to sit at a relatively low, fairly stable baseline. after ovulation, rising levels of progesterone cause a small but measurable rise in basal body temperature, typically by a few tenths of a degree, which tends to stay elevated for the remainder of the luteal phase, until either a period begins or, in the case of pregnancy, temperature often remains raised.
how to track bbt
because the temperature change involved is small, accuracy and consistency matter a great deal. some general guidance:
- use a basal body thermometer, which measures in smaller increments than a standard thermometer
- take your temperature at roughly the same time each morning, ideally after at least a few hours of continuous sleep
- measure before getting out of bed, talking, eating or drinking anything
- record the reading daily, either on paper or using an app or chart designed for this purpose
what the pattern can tell you
because bbt rises after ovulation rather than before it, this method is most useful for confirming that ovulation has already happened in a given cycle, rather than predicting it in advance. over several cycles, a bbt chart can help you understand your typical pattern, including roughly how long your luteal phase tends to last.
things that can affect accuracy
many everyday factors can influence bbt readings and make them harder to interpret, including:
- poor or disrupted sleep
- alcohol the evening before
- illness or fever
- travel across time zones
- certain medications
- taking the reading at a noticeably different time than usual
because of this, bbt tracking works best when looked at as a pattern over several cycles, rather than relying on any single day's reading.
using bbt alongside other signs
many people combine bbt tracking with other signs, such as cervical mucus, to build a fuller picture of their cycle, since no single method is perfectly precise on its own. this combined approach is sometimes used by people trying to conceive, avoid pregnancy, or simply understand their own cycle better.
if you are using bbt tracking for family planning and have questions about how reliable it is for your situation, or if your cycles seem irregular, a healthcare professional can offer guidance suited to you.