Red temperature linechart on pink background with ovulation marked. The word 'Hormones' is written in white text and 'Basal Body temperature' in red.
Home/Cycle Matters / Reproductive Health

Ovulation and temperature: find your fertility

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Written by Lauren McKay

Lauren McKay

Lauren McKay is a writer and journalist with more than ten years of experience writing across a variety of topics. She is a passionate advocate for driving women’s health knowledge and is a trained yoga teacher. She earned a Masters in Creative Writing from the University of Glasgow and currently lives in Scotland.

Medically reviewed by Jack Pearson, Medical Affairs Manager at Natural Cycles

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.

In this post, we’ll look at the link between your body’s temperature and the menstrual cycle, how basal body temperature changes around ovulation, and how tracking basal body temperature can help you either prevent or plan pregnancy. Read on to find out more about this natural method, whether you’re looking to get pregnant or avoid pregnancy.

This article is also available in Spanish.

Temperature and ovulation

Did you know there’s a link between the menstrual cycle and your body temperature? Basal body temperature (BBT) is the body’s lowest resting temperature. A change in hormones around ovulation causes this to rise. Measuring BBT to identify when you ovulate gives you unique insights into your fertile window, in turn giving you increased control over your fertility. There are many things that can affect the timing of when you’re ovulating, so relying on calendar dates to know where you are in your cycle may not be as exact. 

Why measure BBT?

Measuring your temperature when you’re feeling healthy may sound strange. However, measuring BBT is the most reliable way to confirm where you are in your cycle and can help you to predict ovulation. When using this method of natural family planning, you must plan when to time sex if you’re hoping to conceive, or when to abstain for pregnancy prevention.

Other methods such as counting cycle days, tracking cervical mucus, or taking ovulation tests can only give an indication of when you ovulate, whereas measuring BBT can show a definite rise in temperature after it’s happened. It can also help you to detect pregnancy before you even take a test. After ovulation, your basal body temperature rises due to the release of progesterone. If you become pregnant, progesterone will continue to be released to help the fetus develop, meaning your temperature will remain elevated.

How to measure basal body temperature

To measure your temperature in this way, you will need a basal body thermometer. This is a specific kind of digital thermometer that shows two decimal places. You can usually find these at the pharmacy, you can also order BBT thermometers online, or you can get one for free when you sign up for a NC annual subscription

Once you have your special thermometer, you need to take your basal body temperature first thing in the morning before you get up and out of bed. This is because you need to measure the body’s lowest resting temperature. We recommend aiming to measure at least five times a week. 

What’s a normal ovulation temperature?

Before you’re ovulating, a woman's average resting temperature is between 97°F (36.1°C) and 97.5°F (36.4°C). After ovulation, there’s a slight increase in average BBT to between 97.6°F (36.4°C) and 98.6°F (37°C). If a woman is pregnant, this temperature rise will remain higher. However, if a woman isn’t pregnant, it will drop again, causing her to get her menstrual period, and so a new cycle begins.

How hormones affect temperature

You might be wondering what causes this mysterious temperature shift...well, it’s all about hormones! During the first half of your cycle, the main sex hormone present is estrogen. This hormone is associated with the things that make us feel good when we’re ovulating and the days leading up to getting your period, i.e. increased energy levels, high sex drive and clear skin. However, it's the luteinizing hormone (LH) that’s responsible for triggering ovulation itself. 

Right before ovulation, progesterone levels also start to rise. After ovulation, estrogen levels drop off. However, levels of the hormone progesterone stay high for the latter part of the menstrual cycle. Progesterone not only causes symptoms commonly associated with PMS, such as sluggishness and irritability, but it’s also what causes body temperature to stay high. 

Progesterone is an important hormone in early pregnancy as it helps with the implantation of a fertilized female egg cell and helps get the body ready for pregnancy. While you’ll see a dip in temperature before your period begins, those who are pregnant may notice their temperature stays higher due to the presence of this hormone.

How daily basal body temperature charting can help you to track your cycle

Tracking your basal body temperature helps you to understand if and when you’re ovulating. When your ovaries release an egg, there is also an increase in progesterone production, and your basal body temperature rises - usually by between 0.5 to 1.0 degrees celcius. With some practice, you can start to understand your cycle. It’s best to monitor your basal body temperature every day without any breaks, for several cycles, to get a more accurate picture of your cycle.

Your basal body temperature usually drops before ovulation and then rises. During the follicular phase of your menstrual cycle (the first phase of the menstrual cycle, which starts with your period and ends with ovulation), your basal body temperature tends to stay quite low. It will usually be between 97 and 98 degrees Fahrenheit during this phase, until just before ovulation when there will be a significant drop in BBT.

During the luteal phase (the second phase of your menstrual cycle, which takes place after ovulation and before your period), your basal body temperature rises by 0.5 - 1 degrees Fahrenheit. That’s because during the luteal phase, the corpus luteum starts to secrete progesterone - and as progesterone levels start to decrease, your basal body temperature will return to normal a day or two before your period starts. If it remains elevated, when your period is due, this is a good time to take a pregnancy test.

By tracking your basal body temperature every day, after a few months, you’ll have a better idea of when you’re ovulating and when you might get your next period - useful whether you’re trying to avoid pregnancy or want to conceive. Traditionally BBT has been charted with pen and paper, but you can now use Natural Cycles, the world’s first FDA birth control app.

How to measure basal body temperature with Natural Cycles

Natural Cycles is an app that uses the basal body temperature method paired with an algorithm that learns your unique menstrual cycle. While old-school basal body temperature charting methods can require a lot of time and are difficult to use, NC is designed to be user-friendly. It also offers greater accuracy than traditional fertility awareness-based methods, such as the rhythm method and other calendar-based methods. 

Take your temperature most mornings when you wake up. Input the temperature reading into the NC app. If you’re using it for family planning, the app will then use your data to calculate your fertility status for the day and tell you whether or not you are fertile. You can use basal body temperature tracking as a birth control method to prevent pregnancy, or to plan pregnancy

We're constantly looking for new ways to improve the measuring experience for our Cyclers, and that also means offering more ways to measure. Instead of measuring orally with a thermometer, you can now measure overnight by wearing an Oura Ring or Apple Watch. These integrations work a bit differently from measuring with a thermometer since they measure throughout the night. However, Natural Cycles is just as effective when used with a wearable device. 

If you’re using Natural Cycles as a birth control method, you must use protection or abstain from sex on fertile days. It’s 100% hormone-free and is 93% effective at preventing pregnancy with typical use and 98% effective with perfect use.

When can you safely have sex using the basal body temperature method?

If you’re using NC to track basal body temperature as a form of hormone-free birth control, you’ll therefore know that you need to plan sex carefully during the days that you’re most fertile. Your app will guide you with this, but generally, you’re at your most fertile a couple of days before your basal body temperature increases. That means if you want to have sex during your red days (your most fertile period), you’ll need to use a backup method of contraception like condoms. Remember, too that barrier methods like condoms are the only way to protect against sexually transmitted infections, so you may want to use these as an alternative methods of contraception alongside the app, depending on your circumstances.

If you’re trying to conceive, then your most fertile days are the best time for you to have sex.

Other than ovulation, what can affect resting body temperature?

Our bodies are living, breathing, organic things so of course they don’t always operate like clockwork! There are a few things that can affect our basal body temperature. These include:

  • Being unwell
  • Being hungover or drinking alcohol
  • Sleeping significantly more or less than usual
  • Some thyroid conditions
  • Menopause
  • Working shifts where your sleep cycles may be affected
  • Environmental factors like the temperature

If you measure basal body temperature with NC, the app is able to detect changes in temperature and can exclude a reading if it is higher or lower than usual. If you have an underactive thyroid, or are going through menopause, you can still use NC, just be aware that you might get given more red (fertile) days per cycle if the app cannot identify your ovulation day - this does not make it any less effective. 

Key things to remember when using the basal body temperature method

Basal body temperature tracking is simple in principle - but there are a few things to remember to ensure you get the most out of the basal body temperature method, whether you want to get pregnant or avoid it. For the most accurate results, you should:

  • Remember to measure your basal body temperature using your basal thermometer at around the same time every morning. You can set an alarm clock if it helps you to remember.
  • Measure first thing in the morning before getting out of bed.
  • Make sure that you’ve had plenty of sleep before taking your body’s temperature. The results can be inaccurate if you’ve had more or less sleep than usual.
  • If you’re using Natural Cycles, enter the reading into your NC° app. You’ll then get your fertility status for the day. This is either green (not fertile) or red (use protection).
  • Use your BBT chart to better understand your cycle, when you ovulate and when you can get pregnant.
  • Use your fertility status to plan when to have sex if you’re trying to get pregnant, or when to abstain (or use another method of protection).
  • Remember that other factors can affect your basal body temperature, such as if you measure it at a different time of day, if you’re on medication, or if you’re suffering from insomnia.
  • Make sure to record any notes about irregularities, such as if you’ve had less sleep, are hungover, or are taking any new medications. If you are using Natural Cycles you can track this in the app, and can choose to exclude your temperature if you’re not feeling 100%.

Choose your fertility goal

Natural Cycles was FDA cleared in the US back in 2018. It offers a hormone-free option for women who also want to learn more about their bodies, it’s designed to be more accurate and easier to use than traditional fertility awareness-based methods. While Natural Cycles can be used as birth control to prevent pregnancy, it can also be used as natural family planning to plan pregnancy if and when you’re ready to start a family. Are you ready to start your hormone-free journey?

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