How to Measure Basal Body Temperature (BBT) | Your Fertility Friend

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Basal Temperature

What is Basal Body Temperature (BBT)?

Tracking your basal body temperature can provide useful fertility information if you’re looking to become pregnant (and even if you’re looking to prevent pregnancy). Often considered a form of fertility awareness family planning, basal body temperature is something every woman can utilize. So, what is basal body temperature?

Basal body temperature (BBT) is your body’s temperature after resting and before starting any movement for the day. Normally collected at the same waking time each day and before you get out of bed, basal body temperature can have small fluctuations that reflect changes in your fertility cycle, specifically those related to ovulation.

During the first half of a woman’s cycle, her body temperature remains lower. But after ovulation, the body creates progesterone, which causes her BBT to slightly increase and signals that she is no longer fertile for the remainder of the cycle.

Following ovulation, her BBT will likely remain higher than the first half of her cycle.

Temperature fertility tracking works by taking your temperature orally or vaginally each morning, then inputting that information into a chart (a process often referred to as “charting”). From there — and with additional data you collect about your body, such as the presence of cervical fluid, cramps, or other menstrual symptoms — you are able to analyze your chart to find the likely date ovulation occurred.

Why track basal body temperature?

Tracking your basal body temperature each day can give you insights to where your body is in the menstruation and fertility cycle. Temperature fertility tracking can help predict ovulation, which is beneficial for women who are looking to get pregnant and need to time sex near their ovulation date, as well as for women who don’t want to get pregnant and need to avoid having sex around ovulation.

If you’re trying to get pregnant, tracking your basal body temperature can also help you determine if conception has occurred, and if you are likely pregnant before even taking a test. And for women who are having difficulty conceiving, tracking their basal body temperature can give them and their doctor big clues as to why they’re not getting pregnant.

How accurate is the BBT method?

There’s no clear data on how accurate tracking BBT is, though many women find success with taking and charting their temperatures. If you choose to track BBT, you should know that the body is not a clock and can often deviate from the rules.

So, when is a BBT chart inaccurate? Because BBT isn’t an exact science and requires an understanding of your body’s fertility cycle, as well as dedication to following the rules and charting every day, there’s room for error. It’s common for a BBT chart to be inaccurate if:

  • You do not take your basal body temperature at the same time every day
  • If you get up and move around prior to taking your BBT, which raises your body temperature and invalidates that day’s results
  • You use a thermometer that is not designed for BBT charting
  • You are undergoing fertility treatments or use fertility medications that impact ovulation
  • You have consumed alcohol the night prior, which can impact body temperature
  • Your body temperature doesn’t noticeably rise after ovulation.

Still, temperature fertility tracking can be a great tool for learning about your fertility cycle, especially if you are pursuing pregnancy.

Tools You Need To Take Your Basal Body Temperature

Unlike other forms of birth control or fertility treatment, using BBT tracking to prevent or become pregnant is fairly inexpensive. There are only three main tools you’ll need to get started:

  • A basal body temperature thermometer
  • A basal body chart, or an app that creates the chart for you
  • A daily alarm to help you collect your BBT on time before getting out of bed

What basal body thermometer should you use?

If you want to pursue BBT charting, it’s important that you specifically find a thermometer designed for this purpose. While regular thermometers show your body temperature with one number after the decimal point (for example, 97.9 degrees) a basal body temperature thermometer is more sensitive and shows your body’s temperature with two numbers after the decimal point (for example, 98.02).

Basal body temperature thermometers are also designed with higher accuracy; while regular thermometers are accurate within 0.2 degrees, BBT thermometers are more fine-tuned to 0.1 degrees of accuracy. This is important because changes in BBT can be very small and difficult for regular thermometers to pick up.

Where to get a basal thermometer

While basal body temperature thermometers are a bit more costly than regular thermometers, they’re just as easy to find. You can purchase them at a drug store or online, and the often cost between $5 and $15.

Because there are many BBT thermometers on the market, just be sure to choose a reputable brand. Some basal body temperature thermometers have additional features, such as memory recall of prior temperatures and fast results.

There are even high-tech BBT thermometers that sync with phone apps to record your BBT straight into your chart.

Finding and Choosing a BBT Chart

BBT charts, also called ovulation charts, are easy to find online. You can easily download and print a chart from a reputable medical or pregnancy website.

If you don’t want to chart your BBT by hand, you can also consider a charting app. Phone apps such as Kindara, Glow, and Natural Cycles all help you record your BBT and analyze your chart throughout and after your cycle.

Many online BBT resources also provide BBT chart examples, so if you’re new to basal body temperature and charting, you can understand how to accurately record your temperature.

How to Measure BBT

If you’re wondering, “How do I take my temperature,” know that taking your temperature each day is one of the easiest parts of temperature fertility tracing. You’ll simply need to take your temperature, either orally, vaginally, or rectally each morning with a BBT thermometer while remaining calm, and before making any major movements.

What is the most accurate way to measure BBT?

To accurately record your BBT, you’ll need to remember these key steps on how to take basal body temperature:

  • Set an alarm for the same time each morning
  • Upon waking, do not talk, make drastic movements, or get up to use the bathroom
  • Take your temperature orally, vaginally, or rectally while remaining calm
  • Record the results in your BBT chart as soon as possible

Many fertility specialists believe that taking your BBT rectally provides the most accurate information, though don’t feel pressured to do so if you are not comfortable with it. Using your thermometer vaginally or orally still provides accurate results.

When should you chart your BBT?

While it’s best to record your BBT as soon as you take your temperature, it’s OK to head back to sleep after taking your temperature so long as you have a thermometer with memory recall. If your thermometer does not do so, you’ll want to record the information immediately.

For the most accurate BBT chart, get into the habit of inputting the information as soon as possible so that you don’t forget that day’s temperature.

Using a BBT Adjuster

One of the downsides of tracking your basal body temperature is that you’ll have to aim for the same time each day to have consistent results. But sometimes, you oversleep, forget to set an alarm, or wake up earlier than normal. In these cases, your BBT may be higher or lower and can cause inaccuracies in your chart.

If you miss your normal BBT time, it’s important that you still take your temperature. You can simply use a basal body adjuster app to make adjustments. These apps use calculations to estimate what your BBT should have been, based on the understanding that BBT raises about 0.1 degrees every 30 minutes.

Should you use a BBT adjuster? It’s a personal preference, and adjusters are easy to find. Still, many fertility specialists recommend against BBT adjusters, because they can skew your chart and cause inaccuracies.

It’s important that if you choose to use or not use an adjuster, that you be consistent in using it each time or not at all, and that you don’t rely on it regularly.

What should my Basal Body Temperature be?

No two women have the exact same BBT chart, and you’ll notice that your own BBT may fluctuate from prior cycles, during times of illness, or based on outside factors such as sleeping in a warm room. Generally, a normal basal body temperature prior to ovulation is between 96 and 98 degrees. After ovulation, this can range from 97 to 99 degrees.

Temperature fluctuations on a BBT chart

The whole point of charting your BBT is to watch for temperature fluctuations. These minor changes can be major clues into changes with your body.

While normal basal body temperature prior to ovulation is somewhere between 98 and 98 degrees, you may notice that this number fluctuates on a daily basis. For most women, BBT will remain higher during the second half of their cycle (after ovulation) than it was in the first half, signally that ovulation has occurred.

And for women who have abnormally high temperatures, it’s an indication that they may be pregnant. Basal body temperature, if you conceive, is normally high for at least 18 days, giving you a clue that you may have successfully ovulated and gotten pregnant.

How does BBT chart change after ovulation

As your body prepares for ovulation during the first half of your cycle, your BBT remains relatively low. Right before ovulation, many women notice a dip in their BBT.

One hint through basal body temperature ovulation has occurred includes watching for that dip followed by a rise in temperature (caused by progesterone created after ovulation).

So, how long after ovulation does BBT rise? It’s fast — your BBT will rise within 24 hours after ovulation and stay higher than the first half of your cycle until the last days of your cycle, where it may wane.

What if I’m not ovulating?

Tracking BBT is one way to find out if your body is not ovulating, and you can pass this information along to your doctor to determine why you may have difficulty getting pregnant. Though, you should know that if you want to use BBT in conjunction with other forms of hormonal birth control — such as the pill, ring, patch, injection, or implant — you won’t have accurate results.

That’s because BBT is based on tracking signs of ovulation, and hormonal birth control prevents ovulation from occurring, meaning minor changes your body would undergo in preparation of ovulation that impact BBT won’t occur.

How to Detect Pregnancy on Your BBT Chart

If you’re trying to get pregnant, tracking your BBT can be helpful. Generally, your basal body temperature after conception will rise, and stay high.

Most fertility experts agree that a rise lasting at least 18 days after ovulation signals pregnancy. If you’ve been charting for some time and suspect you have conceived, your basal body temperature when pregnant may be higher than previous pre-ovulation temperatures were.

Still, some women find they have no temp rise after ovulation but are pregnant. This can occur because not all women have a major BBT rise following ovulation, which can make it difficult to determine when ovulation occurred.

Other Things to Track

If you plan to use BBT as a way to get pregnant, or even prevent pregnancy, it’s important to track other bodily symptoms. These can help you determine if you may be pregnant, or help you understand when you are fertile if you are avoiding pregnancy. Other things to track on your BBT chart include:

  • Presence of cervical mucus
  • Cramps or ovulation pains
  • When you have sexual intercourse and whether you used protection
  • Period information, such as dates of menstruation and amount of flow
  • Stress, sickness or other health conditions that can impact your fertility

What are the Safe Days to Have Sex When Using the BBT Method?

If you’re using the BBT method to get pregnant, you’ll want to time sex around ovulation — up to five days before you suspect you will ovulate, as well as the day of. But if you’re looking to prevent pregnancy with the BBT method, it’s important that you avoid these fertile days.

According to Planned Parenthood, the safe day to have unprotected sex begin after your temperature has remained elevated at least 3 days following ovulation. However, you can still use the BBT method in conjunction with other forms of birth control (such as condoms) to have safe sex throughout your cycle.