How Basal Body Temperature Charting Works | Your Fertility Friend

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Basal body temperature chart

What is basal body temperature?

Basal body temperature, sometimes referred to as BBT, is the lowest body temperature you reach at rest on a given day. While that technically happens during sleep, it is estimated by taking your temperature immediately after waking up and before you get out of bed.

Basal body temperature is influenced by hormones and fluctuates through the month for women, in correlation with her menstrual cycle. Since basal body temperature is an indicator of where a woman is in her menstrual cycle, it can be used to predict ovulation.

Is there a difference between a basal thermometer and regular?

Basal thermometers do differ from a regular thermometer in that they are much more sensitive than the traditional thermometer. This feature makes it better for tracking ovulation via body temperature.

Whereas a regular thermometer’s main purpose is to detect a fever due to sickness, which is a relatively large change in body temperature, a basal thermometer needs to be more accurate to achieve its purpose of detecting slightly varying body temperature to predict ovulation.

You can purchase a basal thermometer for around $15 online or at most stores that carry pharmacy items. In addition to being more accurate, they are faster and easier to use than traditional thermometers.

Some versions are programmed to save the data for you if you are not able to chart it immediately.

How does basal body temperature charting work?

Charting your BBT is a fertility tool aimed to help you get pregnant more quickly by identifying your most fertile days. It is an easy and inexpensive way to aid in conception or help prevent pregnancy (though it is not recommended as frontline birth control).

Charting may also be recommended by your doctor as a way to identify your menstrual cycle patterns to determine if fertility issues may be at play in your journey to conceive.

BBT charting works by using your basal body temperature to indicate when ovulation occurs throughout the month. Women experience a drop in BBT before ovulation, triggered by a dip in progesterone levels.

By tracking basal body temperature over time, the pattern of when ovulation occurs (indicated by a drop in BBT) will begin to emerge. You can use this information to:

  • Identify when you tend to ovulate each month
  • Time sex for getting pregnant or avoiding pregnancy
  • Detect potential fertility problems related to ovulation or the time between ovulation and your period
  • Assist your doctor in making a fertility problem diagnosis
  • Detect early signs of pregnancy (more research is needed to confirm BBT charting as a tool for this)

How to choose a BBT Chart

The most commonly used basal body temperature charts are software based (computer or mobile). Many women prefer this method as it’s easy, accurate and less likely to fall victim to human error.

These tools are called a fertility calendar or fertility awareness software. There are many fertility websites and apps that offer free tracking tools.

Take Charge of Your Fertility (Harper Perennial, 1995) is considered by many as the go-to source for guidance on charting basal body temperature. You can find sample charts in this book and easily set them up yourself in a simple computer program.

Thirdly, you may decide to create your own graph, plotting temperature on the Y-axis in one-tenth of a degree increments and days of your cycle along the X-axis. You may also use a printable basal body temperature chart.

Regardless of the method you choose, the most important factor in making your decision is picking something that’s easy and accessible for you to use—the simplest method is often also the best.

How to read a basal body temperature chart

As you input data in the form of daily BBT measurements into your charting tool of choice, the dots (data points) connect to each other to create a line with peaks and valleys (rises and dips in temperature).

At the beginning of your menstrual cycle is the Follicular Phase, which, as the name indicates, is when follicles develop. Temperature during this time is normally low and steady. The dominant hormone during this phase is estrogen.

At midcycle you’re likely to see a small dip in BBT followed by a steady rise of about one degree. The day preceding the day your temperature rises is ovulation day. When trying to conceiving this is the point you’re most interested in identifying.

The second phase, which normally lasts around 14 days, is called the Luteal Phase. During this time, the eggs moves down the fallopian tube and fertilization or implantation may happen. BBT during this phase is normally high and regular. Progesterone is the hormone most actively in play.

As you being filling out your chart you’ll want to look for patterns. Looking over three months, for example, you see that ovulation happened on days 12, 13 and 15. If you are trying to get pregnant, you’ll want to have sex during days 6–16, and especially on days 12–15.

You do not need to have sex the day of ovulation to get pregnant. Having sex a few times during the days before ovulation often gets the sperm to the egg in enough time to fertilize it. Most couples opt for having sex every other day during the week before ovulation is expected.

How do you measure BBT?

Because ovulation is indicated by such a small dip in basal body temperature, it’s imperative to measure BBT using the correct method. That is, using a basal body temperature thermometer and taking your temperature first thing in the morning before getting out of bed.

The most difficult part of charting basal body temperature is remembering to do it every day, first thing in the morning, before you move. Even getting up to go to the bathroom can raise body temperature and lead to an inaccurate reading.

When should you start charting?

BBT charting should ideally begin the first day of your period.

What tools do you need to take your basal body temperature?

A basal thermometer is needed to take your basal body temperature. It is not the same as a regular thermometer in that it captures a more accurate reading of body temperature.

Basal thermometers are able to detect even minute changes in body temperature, which is essential when charting your basal body temperature, as the indicator of ovulation is a very small jump in this measurement.

What should my basal body temperature be?

BBT is not the same from person to person. Unless abnormally high or low, your basal body temperature has no influence on whether or not you conceive. Rather, the tracking of the fluctuations in your own BBT is what matters for charting.

That being said, most women have a basal body temperature between 96 and 98 degrees before ovulating and a BBT of 97 to 99 degrees after. You can expect your BBT to rise only about one-tenth to one-half degree after ovulation.

What is your BBT during period?

Your basal body temperature will drop about 10–16 days after ovulation when levels of progesterone drop and you start your period. Your may see another drop in temperature when you get your period.

How to Identify Ovulation

A minor spike in your basal body temperature indicates ovulation. The “spike” ranges anywhere from a one tenth to a one half degree difference.

Since the change is so minor, it’s imperative to use the right tools—a basal body temperature thermometer—and take your temperature the correct way (See How do you take your basal body temp when ovulating?) to collect an accurate reading each day, in turn predicting accurately when ovulation will occur.

You should take your basal body temperature every day and log it to keep a record. After a month of two, a pattern will emerge that shows when ovulation happens.

How do you take your basal body temperature when ovulating?

When charting your basal body temperature to track ovulation, you should take your temperature using a basal thermometer early in the morning before getting out of bed.

To get an accurate reading, it’s imperative that your take your basal body temperature before moving at all as by definition it is your temperature when at rest.

Even small movements can cause your temperature to rise and throw off an accurate reading. You will use the same method of taking your temperature during ovulation as any other day.

What is a normal BBT before ovulation?

A drop in temperature of more than 0.4 degrees Fahrenheit (-17.6 degrees Celcius) happens 48 hours before ovulation. During this time, a normal basal body temperature is less than 98 degrees Fahrenheit (36 degrees Celcius).

Does your BBT rise after ovulation?

Basal body temperature rises after ovulation, typically rising above 98 degrees Fahrenheit (36 degrees Celcius).

How many days after ovulation does your BBT stay elevated?

Your basal body temperature will not rise until 12–24 hours after ovulation. For women who are not pregnant, BBT will continue to rise for 10–16 days before starting your period.

What is cervical mucus?

Some woman also track changes in their cervical mucus to help indicate when she is most fertile. Throughout your menstrual cycle, the color, texture and amount of cervical mucus varies due to changing hormone levels.

Following your period, there may be no discharge for a few days followed by a few days of cloudy, sticky discharge. In the few days before ovulation, discharge is thin, stretchy and slippery (like egg whites) and increases.

This stage makes it easier for sperm to travel in the cervix and fertilize the egg. You are most fertile on these days. Following ovulation the discharge becomes thicker and decreases.

A good time to check your cervical mucus is first thing in the morning when you go to the bathroom, after charting your BBT. Unlike measuring basal body temperature, checking cervical mucus can be done at any time of day.

Can basal body temperature indicate pregnancy?

BBT may be an indicator of pregnancy. Using basal body temperature charting to predict early pregnancy is part of the never-ending quest for early pregnancy confirmation. Like with other methods, it is impossible to confirm pregnancy using this method alone.

What is your basal body temperature when pregnant?

A small spike in BBT can be one of the first signs of pregnancy. Your basal body temperature will rise as much as one degree when pregnant.

When you become pregnant your body releases more progesterone, a hormone that causes the spike. As your pregnancy progresses, your body temperature will lower back to normal levels.

Implantation dip on body basal body temperature charts

An implantation dip refers to a one-time dip in BBT roughly a week following ovulation. Because of where it shows up in you cycle, most of the time an implantation dip is simply a mid-cycle drop in temperature and does not indicate conception. Because of this, it’s debatable as to whether or not this is an early pregnancy sign.

Does your BBT drop after implantation?

While there is no exact explanation of why, basal body temperature is known to drop upon implantation. If an implantation dip does occur, it’s quickly followed by a rise in BBT as progesterone is released and warms the body.

Is implantation dip a reliable indicator of pregnancy?

While an implantation dip in BBT is seen more often in a basal body temperature chart of someone pregnant vs. not pregnant, it does not necessarily mean you are pregnant.

Basal body temperature chart examples

There are a variety of basal body temperature charts to choose from, including everything from basal body temperature charts for early pregnancy to basal body temperature charts for menopause. A quick internet search will result in a plethora of options to explore. Here are two places to start:

Whichever chart your choose, the key is to make it simple to track and easy to read.