If you’ve started looking into trying to conceive or downloaded a fertility app, you’ve probably seen the term “basal body temperature” and its abbreviation, BBT. It refers to your body’s lowest temperature when at rest. When you ovulate, your BBT rises slightly – about half of a degree Fahrenheit.

Your most fertile window of time is the two days before that temperature rise. So tracking BBT to predict ovulation as a method for pinpointing your most fertile time has long been used in natural family planning.

You can get started tracking by choosing an oral digital thermometer that is precise enough show differences of 1/10th of a degree. It’s easiest just to purchase one that is specifically marketed for basal body temperature. You can buy one online or in any drug store for about $10.

There are three important things to remember when tracking your BBT. You should take your temperature:

  • first thing in the morning before rising
  • at the same time daily
  • preferably before 7:30 am

You can log the temps over time and pinpoint any variations using a pen and a calendar, but fertility-tracking apps make it easier.

If you see variations, keep in mind that some factors can affect your BBT, such as:

  • taking temperature at a different time than usual
  • excessive alcohol consumption the night before
  • illness
  • using a different type of thermometer or method
  • altitude change
  • less than an hour of sleep just before waking
  • unusual chill or heat
  • fever
  • taking temperature after 7:30 am

And once you succeed in conceiving, your basal body temperature may be one of the first indications. It may show a slight increase for the first 18 days or more.

To learn how to interpret your BBT and other fertility signs, sign up for Conceive, a 90-minute class from Well+Prepared.


Well+Prepared is a pre-pregnancy program from Baby+Company. Get expert consultation, informative classes, and 1-on-1 coaching to help you conceive a healthy pregnancy. Well+Prepared is available in all Baby+Company centers. Find the center near you:

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