Three-Day Rule (BBT)

The three-day rule is a charting rule for basal body temperature: the fertile time counts as over after three mornings in a row of raised waking temperature. Temperature rises about 0.3 degrees Celsius after ovulation, because progesterone rises.¹

The rule exists because the fertile time is short. A woman can conceive from one act of intercourse on only about seven days of a cycle. The chance falls to zero within 24 hours after ovulation.¹ Three high readings build in a margin for a temperature thrown off by illness or a short night.

A temperature rise shows that an ovulatory event happened. It does not prove an egg was released. Mucus patterns point to the fertile days before they arrive, which is why sympto-thermal methods read both signs.