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Getting Pregnant

How to Use Basal Body Temperature to Determine Ovulation

The Temperature Method: A Good Starting Point

If you want to determine when in your cycle you ovulate, tracking your basal body temperature (BBT) is a good starting point. BBT is the lowest temperature your body reaches in a 24-hour period. In women, BBT follows a biphasic pattern throughout the menstrual cycle—that is, it’s lower in the first half of the cycle. After ovulation, BBT rises by about one half to one degree Fahrenheit and stays elevated until your next period.

By taking your BBT every day and keeping track of the results, you can estimate when ovulation might have occurred.

Why the Temperature Method Works

After ovulation, your ovaries secrete the hormone progesterone, which stimulates the production of your uterine lining (in preparation for the possible implantation of an embryo). In addition to stimulating the growth of the uterine lining, progesterone also causes your BBT to rise.[1.Your BBT hits a nadir a few days before you ovulate, but many women are unable to detect this in their charts.] Your BBT rises after ovulation and stays elevated until just before your next period. (If you get pregnant, it will stay elevated all the way until the end of the first trimester.)

How Accurate is the Temperature Method?

While the vast majority of women experience a BBT nadir before ovulation and a BBT rise after ovulation, these changes don’t always happen on the day of ovulation itself. Only eleven percent of women experience a BBT rise within a day of ovulation, and only 40 percent of women experience a BBT nadir within a day of ovulation.

Is the Temperature Method Right for You?

Many women believe that BBT charting can help you time intercourse around conception. But for many women, this simply isn’t true.

Your fertile window is the five days before ovulation and the day of ovulation itself. But your BBT changes 12 – 24 hours after ovulation. Since the egg survives for no more than a day after it is released, by the time your BBT indicates ovulation, there is little fertile time left to conceive.

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Because of this, BBT charting is best for women with very regular cycles, who can use the previous month’s BBT chart to predict the fertile window for the following month.

How to Get an Accurate BBT Reading

  • First, make sure you’re using a BBT thermometer. Regular fever thermometers aren’t sensitive enough to detect the small rise that occurs after ovulation.
  • You can take your BBT in your mouth, or for greater accuracy, your vagina. Whichever orifice you decide on, make sure you’re consistent.
  • Keep your thermometer on your bedside table and take your temperature as soon as you wake up—before you talk, drink water, sit up, or go to the bathroom.
  • Take your temperature at the same time each morning, give or take a half hour.
  • You need at least three hours of consecutive sleep to get an accurate reading, so if you get less than that, don’t include that day’s results.
  • Alcohol, drugs, poor sleep, and illness (especially fever) can impact your BBT. If you had a lot to drink, slept very poorly, or you’re sick, you may want to skip taking your temperature that day.

Beyond the Temperature Method

If you’re looking for a more precise and accurate way to track your fertility, consider the Ava bracelet. Ava is worn at night, so you don’t have to worry about waking up at the same time each morning. While you sleep, the sensors on the bracelet are continuously tracking not only your temperature, but also your heart rate, sleep quality, and more. Together, this information enables Ava to detect an average of 5.3 fertile days per cycle in real time.

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Lindsay Meisel

Lindsay Meisel is the Head of Content at Ava. She has over a decade of experience writing about science, technology, and health, with a focus on women's health and the menstrual cycle. Her work has been featured on The Fertility Hour, The Birth Hour, The Breakthrough Journal, and The Rumpus.

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