Basal body temperature (BBT): definition and meaning

How it works in your cycle

Before ovulation, BBT sits in a lower band. Once the egg is released, the corpus luteum makes progesterone, which nudges your thermostat up. Three higher readings in a row usually confirm ovulation has passed. If you're pregnant, the rise stays; if not, it drops around your period.

1. Take your temperature at the same time each morning, before getting up.
2. Use a thermometer with two decimal places.
3. Chart it daily and look for a sustained shift, not single days.
4. Note things that skew it: illness, alcohol, poor sleep, travel.

Why it matters

BBT is useful for learning whether and when you ovulate, and for spotting a short luteal phase. Its limit: it rises after ovulation, so on its own it can't predict your fertile window in the current cycle. Wearables like wrist temperature on Apple Watch track a similar overnight shift.

The higher-temperature days line up with the luteal phase, which is when many people notice PMS mood changes. Seeing both patterns side by side can be reassuring. Full guide: what is basal body temperature.