A Guide to Basal Body Temperature and Ovulation (+BBT Chart)

by Nov 25, 2020

The basal body temperature (BBT) method is a natural fertility planning method that can be used by women who are trying to conceive (TTC) and trying to avoid (TTA) pregnancy to predict ovulation. The method has been around a long time — since the 1930s, in fact — so there is a lot of evidence behind how basal body temperature and ovulation work together.

A woman taking sun baths

However, the BBT method relies on accuracy, and beginners often make mistakes when using it for the first time. Learning how to use basal body temperature to determine ovulation is important for an accurate estimate of your fertile method. Read on for helpful tips and facts you should know before trying it at home.

What is basal body temperature?

Your basal body temperature (BBT) measures your body temperature when you are completely at rest. There is a slight rise in BBT after ovulation, usually less than two degrees Fahrenheit. Women can use this increase in BBT to predict when they are most fertile, to either conceive or avoid sex during that time.

Before ovulation, a woman’s average BBT is between 97 and 97.5 degrees Fahrenheit. After ovulation, a woman’s average BBT is between 97.6 and 98.6 degrees Fahrenheit. This BBT chart shows how a woman’s BBT can change after ovulation:

BBT Chart

As you can see, the shift in BBT after ovulation is very subtle — in fact, BBT may change by as little as 0.1 degrees Fahrenheit!

A common mistake that beginners make with the BBT method is confusing BBT with normal body temperature. However, unlike normal body temperature, BBT can only be measured first thing in the morning, before you have gotten out of bed, using a special thermometer. BBT thermometers measure temperature in tenths of a degree, allowing women to track very slight changes in their body temperature.

BBT must be measured first thing in the morning because even small movements — like walking from your bed to the bathroom to brush your teeth — can cause slight increases in body temperature. Thus, for the most accurate results, you should take your BBT when still lying in bed. Many women find it helpful to keep the BBT thermometer next to their bed at night to remind them to take their temperature as soon as they wake up the next morning.

Basal body temperature and ovulation

Before ovulation, there is a surge in a reproductive hormone called luteinizing hormone, or LH. This LH surge triggers the release of an egg by the ovary, a.k.a. ovulation. When LH surges before ovulation, progesterone levels also begin to rise, and BBT rises along with it.

Changes in BBT become most noticeable in the days following ovulation. The increase in temperature after ovulation lasts 10 to 17 days in a part of the menstrual cycle known as the luteal phase.

It’s important to note that your most fertile days take place two to three days before your BBT rises. So, BBT can confirm that your ovulation has occurred, but it does not predict when you will ovulate. This limits its accuracy for preventing pregnancy but does help you track and monitor ovulation if you’re TTC.

How to track your BBT to estimate ovulation

Tracking your ovulation by temperature requires advance preparation. The BBT method requires you to buy a special BBT thermometer and measure your temperature every day before you get out of bed in the morning.

You will also need to select a type of BBT chart to record changes in your BBT over time. Some women track their BBT with an app while others prefer to use graph paper. Choose whatever BBT charting method works best for you.

To track your basal body temperature, you’ll need to:

  • Take your temperature every morning. It should be taken at the same time each day.
  • Take your temperature before rising out of bed.
  • Use a BBT thermometer orally or vaginally. Use the same method every time.
  • Record your temperature using your basal body temperature chart.
  • Repeat the process for at least three cycles.

How accurate is the basal body temperature method?

The BBT method is popular for a reason: it can reliably confirm when ovulation has taken place, as long as you use it accurately and consistently. Forgetting to take or chart your BBT for just one day could cause you to miss your ovulation. This could result in an unplanned pregnancy if you are TTA.

However, BBT is only an estimate, and it cannot predict ovulation before it has occurred. To predict ovulation before it happens, you’ll need to track your levels of LH before and after ovulation to observe when an LH surge takes place.

Without tracking your LH levels, you will only be able to estimate when you are going to ovulate next. This can be problematic because there is sometimes a great variation between ovulation days during different cycles, even in the same woman! As a result, you might get pregnant, even though you are TTA.

There are lots of studies on the pros and cons of the BBT method:

  • A study among six physicians recorded basal body temperatures of three women. Out of their collective charting, the accuracy rate was 38.1%.
  • One study of an app used to track BBT for pregnancy prevention found a typical-use failure rate of 8.3%, making it more effective than other fertility awareness methods.
  • In another study with 30 women, BBT charting had a failure rate of 20% when estimating ovulation.
  • In 2005, a medical review article concluded BBT was not an accurate method for predicting ovulation.
  • New evidence suggests that wearable technology measuring wrist skin temperature may improve the accuracy of the BBT method.

There are even more factors that can cause the BBT method to fail. For example:

  • You may not have time for consistent daily tracking.
  • Having a fever will make tracking inaccurate.
  • Being emotionally or physically stressed can raise your body temperature.
  • Consuming alcohol, drugs, and prescription medication can cause temperature changes.
  • Women do not always have consistent increases and decreases in BBT.
  • Women have unique menstrual cycles that may make charting difficult.

These factors show that BBT can be a difficult method to follow and use consistently. A BBT chart is not always the best or most reliable way to predict ovulation or avoid pregnancy. However, it can be helpful, especially in combination with other tracking methods, like using a digital fertility tracker.

Other ways to track your cycle

There are many other natural fertility planning methods you can use with or without the BBT method to prevent or plan for pregnancy:

  • Ovulation calculators. Ovulation calculators give a simple estimate of when you ovulate based on the length of your cycle in calendar days. The formula used is to calculate the date of ovulation is (day of your next period – 14 days = ovulation day). The formula used to calculate your fertile window is (day of ovulation – 4 days = start of your fertile window). However, ovulation calculators are flawed because every woman’s cycle is different, and ovulation days may vary from month to month.
  • Cervical mucus method. Many women use the cervical mucus method alongside the BBT method to track their fertility. Cervical mucus is the slight discharge you may notice in your panties when you are feeling normal and healthy. Throughout most of the month, your cervical mucus is thick, creamy, and white or off-white in color. When a woman is most fertile, however, the cervical mucus becomes clear and stretchy, like an egg white. You can collect and observe the changes in your cervical mucus using a finger, a piece of toilet tissue, or a pantyliner, but because many things can affect your cervical mucus besides fertility — such as infections and sexual activity — you may want to refrain from relying solely on the cervical mucus method.
  • Ovulation predictor kits (OPKs). OPKs are drugstore tests you can purchase that will give you a positive or negative result to tell you if you are currently ovulating. They work the same way an at-home pregnancy test does, by measuring hormone concentrations in your urine. OPKs measure your LH levels and compare them to a threshold drawn from population averages to let you know if you are ovulating. The best test kits are 99% accurate, but sometimes give false negatives or false positives if your unique hormone levels differ from the population average. You must also test frequently to get the most reliable results, which can get expensive over time.
  • Digital fertility trackers. Digital fertility trackers like Mira improved upon OPKs by measuring LH levels in your urine with laboratory-grade accuracy. Because it relies on artificial intelligence to make smart predictions about your cycle, Mira can detect ovulation even when your LH surge is small or when you have many smaller surges in LH throughout your cycle (which is often due to a hormonal condition like polycystic ovarian syndrome). Mira also learns when you do and don’t need to test and will send you reminders, saving you money on test strips and making sure you don’t forget to track your hormones.

Where to start with BBT when it comes to ovulation

When used consistently and correctly, the BBT method is one of the more accurate fertility awareness methods (FAMs) for preventing and planning pregnancy. However, like all FAMs, the BBT method is most accurate when used in conjunction with other tracking methods. Using BBT along with other metrics to predict ovulation is known as the symptothermal method. Combining the BBT method with other data like cervical mucus changes or fertility hormone concentrations can help you get pregnant faster or avoid pregnancy more reliably than using BBT alone.

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