Basal body temperature, explained

Your basal body temperature is your body temperature at full rest. After ovulation, progesterone gives it a small nudge up, and it stays higher until your next period.

How to take it

1. Use a thermometer that reads to two decimal places (a BBT thermometer).
2. Take it every morning as soon as you wake, before you sit up, get out of bed, eat or drink.
3. Try to measure at about the same time each day.
4. Log it in Splash and look at the pattern over the whole cycle.

Reading your chart

You are looking for a shift: a run of lower temperatures, then a sustained rise that lasts until your period. The rise shows ovulation has already happened, so BBT is great for confirming ovulation after the fact, but it cannot predict it in advance. ACOG says the most fertile days are the 2 to 3 days before the rise.

Things that throw it off

• Illness or fever. ACOG warns BBT may not be reliable if you are unwell.
• Broken sleep or a lie-in.
• Alcohol the night before. In the large app study, cycles flagged for fever or high alcohol intake were handled separately.
• Travel and time-zone changes.
• Hormonal contraception that stops ovulation will also flatten the pattern.

What about wearables?

Some wrist, finger or ring devices measure skin temperature overnight. In a study of 237 women trying to conceive, a wrist bracelet combining skin temperature, pulse and other signals could identify the fertile window with reasonable accuracy. But that is one study of one device, and skin temperature is not quite the same as a mouth temperature, so treat this as promising rather than proven.