Basal Body Temperature -  How to Check BBT to Detect Ovulation

Basal Body Temperature - How to Check BBT to Detect Ovulation


What is Basal Body Temperature (BBT)?

Basal body temperature (BBT) refers to your lowest body temperature when you are at complete rest, and it changes depending on a number of different factors that include your hormonal changes.

Under normal circumstances, when you are not pregnant, your progesterone levels rise during ovulation and remain high for about two weeks after that. Then, as you menstruate, your progesterone levels drop, causing a fall in your body temperature. Women are considered to be most fertile in the 2-3 days prior to this temperature increase and tracking it monthly can help you make an educated guess about when to have sex so that you have the highest chance of conceiving.

BBT charting helps you in determining your ovulation time, which can aid in getting pregnant. But, you need to monitor your temperature for around 2-3 cycles to figure out a proper ovulation pattern.

Why measure BBT and not normal body temperature?

As you go about your day, a lot of factors ranging from stress, cold or heat, exercise and diet can affect your normal body temperature. But, after you have received at least 4-5 hours of proper rest and have not yet engaged in any physical activity, your temperature remains unaffected by external factors and is considered to be most reliable. This temperature is the most ideal to indicate ovulation.

Why should you track your BBT?

Tracking your BBT and maintaining a comprehensive chart comes with its set of advantages like:

  • Understanding the bodily changes that are going on inside you.
  • Determining on which days you are most likely to be fertile.
  • Predicting when your period is supposed to come.
  • Finding underlying hormonal or reproductive issues.
  • Determining when to refrain from sex so as to avoid conception.

How to check basal body temperature?

Even though the method of checking and keeping a track of your BBT is simple, it still requires a level of commitment that you must make in order to get accurate results.

  • Check your temperature the first thing every morning before you engage in any other activity.
  • Track your temperature around the same time every morning.
  • There are three ways you can check your temperature: orally, vaginally and rectally, and it is important to follow the same method each time.
  • Make sure to use the same thermometer every time you check your temperature.
  • Do not check the temperature unless you’ve had at least 3 hours of sleep.
  • Use an app or a chart to log all your data for at least 2-3 cycles for accurate tracking.

You should note down a few things for a more accurate reading:

  • Rectal reading is considered to be the most efficient when checking your BBT.
  • Do not check your temperature in your underarm region as the reading will not be accurate.

Choosing a BBT chart

There are a number of BBT charting methods that you can opt for and even though using an app or software can minimize human errors, you can also do it manually on paper if that’s what you’re comfortable with. Choose from the following available options and see what works best for you:

  • Use charts that come with fertility books.
  • Use a fertility calendar software available for your computer.
  • Use a fertility or menstrual cycle tracking app that offers the option of logging in ovulation tracking data.
  • Track manually on chart paper by plotting the temperatures along the Y-axis and the cycle days on the X-axis.

How to use the BBT chart to predict ovulation?

There are two methods of tracking ovulation:

  • By visiting a clinic to get an ultrasound done to check for ovulation.
  • By tracking your own basal body temperature and maintaining a chart.

The BBT method acts as a simpler alternative to the USG one, where there is a drop a few days right before ovulation, after which it keeps increasing. The pattern is as follows:

  • Follicular phase - The BBT stays in the lower range during the follicular phase (around 97-98 degrees Fahrenheit) till it’s about a day before ovulation, which is when your BBT dips at the lowest point.
  • Luteal phase - After ovulation happens, your corpus luteum starts secreting progesterone which increases the basal body temperature by 0.5-1 degree Fahrenheit and maintains this temperature throughout the luteal phase. As you begin to approach the end of the luteal phase, your BBT drops to its earlier range on the lower side about a day or two before or on the day of your period.

How accurate is this method for tracking ovulation?

Whether you are planning to get pregnant or using BBT tracking as a method of contraception, it is has a 99% efficacy rate when tracked properly in an ideal setting. However, if there are unfavorable circumstances, or if the tracking has been done incorrectly, the efficacy rate drops down to 75%. You should never use it as the only method of contraception if you want to avoid unwanted pregnancy for this reason.

When are the instances that a BBT chart may be inaccurate?

If you are on hormonal contraception, your BBT tracking will always be incorrect since you have synthetic hormones interfering with your body. However, in certain circumstances, your chart may be inaccurate even when you are not on hormonal birth control:

  • If you vary the time of your temperature tracking or interchange the body part you are tracking from.
  • If you are down with a fever.
  • Certain medications can affect your basal body temperature.
  • If you consumed alcohol the day before tracking.
  • If you are either physically or emotionally stressed.
  • If your body has undergone a major physical stressor that can disrupt its circadian rhythm, like jet lag.
  • You use an electronic heating blanket or device while sleeping.
  • You are suffering from insomnia or a disrupted sleeping cycle.
  • You are engaged in breastfeeding.

Choosing a BBT thermometer

There are two kinds of thermometers you can opt for according to your personal preference:

  • Electronic thermometers - These are easier to use as they signal when your peak temperature is reached and it also has the capability to store all the information for future reference if you happen to need it.
  • Mercury thermometers - These are analogue thermometers that need to be shaken before use. But, keep in mind that you must keep it ready the night before or get someone else to shake it for you in the morning to avoid engaging in any activity as that might disrupt your reading. Also, while using a mercury thermometer, if the reading falls between two markings, make sure to consider the lower one when taking a reading.

Can you detect pregnancy on a BBT chart?

It is possible to tell a probable pregnancy by reading a BBT chart. When the egg is fertilized, it takes about 6-10 days for it to get implanted into the endometrium, and a steep decline in BBT may be noted around this time owing to the release of estrogen. 

Subsequently, as progesterone is released, the temperature begins to rise again and if it remains elevated for more than 14 days after the initial possible denotation of ovulation and your period doesn’t come, it could be indicative of pregnancy.

Post conception, the BBT chart may show a triphasic pattern, making three temperature levels visible, indicating a possible pregnancy. But, this is not a foolproof method of denoting conception, and you should always consider taking a home pregnancy test or opt for a blood test to be sure.

What should your next step be after preparing your BBT chart?

After you’re done tracking one full cycle, move on to the next one and repeat the process for several cycles and keep all the data ready. Follow the chart to determine your fertile days and attempt conception, or use it to avoid pregnancy.

If your cycle appears to be irregular or you have failed to conceive after trying for a few months and everything appears to be normal on your charts, you can opt to speak to a doctor to get advice on what your next steps should be.

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