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What is basal body temperature?

Basal body temperature is the body’s lowest resting temperature, which can be measured as soon as you wake up in the morning. In this article, we’re going to cover the crucial link between measuring basal body temperature and finding your fertile window. We’ll also look at different things that can affect your resting temperature, how to measure basal body temperature, and much more.

This article is also available in Spanish. 

BBT meaning

BBT is a common abbreviation for basal body temperature. If you’re reading up on fertility awareness, you might see references to the BBT method, BBT thermometers or tracking BBT. This is just shorthand for basal body temperature. Understanding your BBT can give you unique insight into your menstrual cycle and give you increased control over your fertility goals. 

How does BBT work?

Basal body temperature is a very sensitive measurement. Unless you know what to look out for you might not even notice a change in body temp! However, there is a link between ovulation and temperature, with a shift in basal body temperature after ovulation happens in the menstrual cycle. This is due to an increase in the hormone progesterone. This shift in resting body temperature can pinpoint where a woman is in her cycle and this knowledge can then be used to find the fertile window (i.e. when you can get pregnant).

How to take basal body temperature

You can measure your basal body temperature using a special thermometer that shows two decimal places. You should do this first thing in the morning before you get up and out of bed as you need to capture the body’s lowest resting temperature. Keep in mind that there are a few things that can affect BBT… we’ll go into more detail on that later!

Reasons to measure basal body temperature

While we might mostly think about measuring temperature when we’re checking for fever, there’s a very good reason to measure basal body temperature on a more regular basis. Keeping track of basal body temperature shifts can tell you exactly where you are in your cycle, something you can’t know just by counting cycle days. 

This precision is useful if you’re looking for a hormone-free way to prevent pregnancy, or are wanting to find the days when you have the best chance of conceiving if you’re trying for a baby. At Natural Cycles we recommend measuring around five days a week to get enough data to confirm the fertile window. 

Can you use a regular thermometer for BBT?

No, you need a BBT thermometer to measure for basal body temperature. This thermometer shows two decimal places. You should be able to find one at the pharmacy, you can order a BBT thermometer online, or if you choose to sign up for a Natural Cycles yearly subscription, you’ll get one included. 

Normal BBT

Everyone is different, so a ‘normal’ basal body temperature doesn’t really exist. However, before ovulation a woman’s BBT averages between 97°F (36.1°C) and 97.5°F (36.4°C). This rises after ovulation and stays high for the second half of the cycle.

What is basal body temperature after ovulation?

After the female egg cell is released at ovulation, BBT increases to between 97.6°F (36.4°C) and 98.6°F (37°C). Basal body temperature will drop again if pregnancy doesn’t happen. This temperature drop causes the uterus lining to shed, causing a period to start, and a new menstrual cycle to begin. 

Other signs of ovulation

A rise in temperature is a clear indicator that ovulation has happened. However, there are a few other signs of ovulation you can look out for. These include a surge in luteinizing hormone (LH) in the days before ovulation is due to happen, increased sex drive, and a change in the consistency of cervical mucus. While these are ovulation indicators and can point to the fact ovulation may be approaching, none of them can confirm ovulation has actually happened. To do that, you need to measure basal body temperature.

Introducing Natural Cycles

Natural Cycles uses the basal body temperature method, but takes it one step further. The app analyzes your temperature data for you and is powered by an algorithm that learns the pattern of your unique cycle to predict ovulation. This removes the difficulties of traditional charting and finds your fertile days more accurately than other counting-based methods, such as the rhythm method. 

If you’re using traditional BBT to determine fertility, you can only detect the rise in temperature after ovulation has happened. This is at the end of the fertile window, which is tricky for both those trying to plan or prevent pregnancy. However, the Natural Cycles algorithm is able to predict ovulation based on your historical data and detect ovulation in real-time. This means not only will you know ahead of time when you're fertile, but you will be able to confirm ovulation has in fact happened.

What affects BBT?

There are a few things besides ovulation that can affect basal body temperature, so it’s good to be aware of them. Not getting enough sleep, or sleeping more or less than you usually do can have an impact on your resting body temperature. Consuming more than a couple of glasses of beer or wine can also lead to an elevated body temperature. Being unwell can also cause a fever, this in turn affects the body’s resting temperature. Natural Cycles is able to detect changes in temperature and can exclude a temperature if it is higher or lower than usual.

Certain conditions such as hypothyroidism can affect BBT, and those going through menopause may also experience fluctuations in basal body temperature due to hot flashes. Keep in mind that Natural Cycles still works with these conditions, but the app may give you a longer fertile window if it can’t identify your ovulation due to a varying temperature.

Basal body temperature when pregnant

One of the signs of early pregnancy is a consistently high basal body temperature. Usually, in the menstrual cycle, temperature dips again right before we get our periods. However, in early pregnancy, temperature stays high. This is due to the raised level of the progesterone hormone, which stops the uterine wall from shedding and helps with the implantation of the fertilized egg cell.

Measuring BBT vs counting cycle days

When it comes to fertility-awareness, not all methods have the same effectiveness, and not all methods work in the same way. For example, methods that work by predicting ovulation based on cycle length do not identify your specific ovulation in the same way that measuring BBT does. This is because counting cycle days can only give an approximation as to when ovulation happens, on the other hand, a temperature reading can confirm ovulation has actually occurred.

In 2018, Natural Cycles became the first digital birth control method to be cleared by the US FDA. It now has more than 3 million registered users worldwide. As well as certified birth control, Natural Cycles can also be used to plan a pregnancy. Are you ready to take control of your fertility?

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Written By

Jennifer Gray

Jennifer Gray is an award-winning writer with more than five years’ experience covering reproductive topics ranging from birth control to planning pregnancy. She is passionate about providing women with accurate information grounded in science they can use to take charge of their own health - while also dispelling myths that exist within the field of women’s health. She holds a Master of Science from the University of Edinburgh and currently lives in Ireland.

Jack in a suit and tie holding a microphone and giving a presentation.

Scientifically Reviewed

Jack Pearson

Dr. Jack Pearson is a previously HCPC registered Embryologist with a PhD in reproductive medicine. Prior to joining Natural Cycles leading Medical Affairs, he worked for more than 10 years in a clinical setting working at some of the busiest fertility clinics in the UK. Today he spends most of his time working with experts at the world’s leading institutions to carry out important research with the vision to further the field of female health. He earned his PhD from the University of Sheffield specializing in Sperm Metabolism and currently lives in London.

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