Basal body temperature: Natural family planning method

This article is also available in Spanish

What is the basal body temperature method?

The basal body temperature (BBT) method — a.k.a the temperature method — is a type of fertility awareness method in which you track your temperature to help determine when you’re ovulating [1]. The idea behind it is simple and based on the amazing science of the menstrual cycle: our body temperature during each cycle follows a clear curve driven by different hormones [2], and we’re only fertile during a six-day window before and on the day of ovulation [3]. 

Measuring your basal body temperature when your body is fully at rest can help you pinpoint where you are in your cycle, and help you find your fertile window (i.e. when you can get pregnant). 

What is the difference between basal body temperature and body temperature?

BBT is the body’s lowest resting temperature; the baseline body temperature you get when you’ve just woken up from several hours of sleep [1]. Your “regular” body temperature, on the other hand, will fluctuate a bit during the day depending on what you’re doing (think of it as similar to heart rate; you can compare your nightly resting heart rate from one day to another, while during the day your heart rate will move up and down depending on what you’re doing at that moment). In short, the difference between your BBT and body temperature lies in when and how you measure. 

When using the BBT method, you can’t just take your temperature at any time during the day — to track the subtle changes during the menstrual cycle, you need to make sure that you measure your BBT first thing in the morning so the temperature isn’t affected by something else.

You’ll also need a special kind of thermometer. While a regular fever thermometer can be used to measure body temperature at any time, basal body temperature requires a more sensitive thermometer that measures two decimal places to use in those crucial morning moments as your body awakens from rest.

Why is the basal body temperature method used?

The BBT method can be used either as a contraceptive method or when trying to conceive. Since finding ovulation allows you to understand which days during your menstrual cycle you’re most likely to be fertile, you can either plan to have sex on your most fertile days to optimize your chances of getting pregnant or know when you need to use protection (or abstain) to prevent pregnancy. 

The motivations for choosing the BBT method for contraception are as many as there are individuals — some may choose this method if they don’t want to or can’t use hormonal birth control, or to learn more about their body and fertility.

How does basal body temperature work?

There is a link between ovulation and temperature, with a small (but significant) shift in basal body temperature after ovulation [4]. Before ovulation, in the estrogen-rich follicular phase, our temperature is slightly lower. Once ovulation happens, the body releases the hormone progesterone, which causes our body temperature to increase and stay elevated during the luteal phase, the second phase of the menstrual cycle. 

Basal body temperature is measured first thing in the morning as soon as you wake up — before you sit up, talk, or get out of bed. Tracking the hormone-driven temperature changes that happen during your cycle using the BBT method allows you to understand when you ovulate and which days you’re fertile. 

What can affect basal body temperature?

Ovulation aside, there are a few things that can affect basal body temperature that are good to be aware of when using the temperature method:

• Sleep: Not getting enough sleep, or sleeping more or less than you usually do, can have an impact on your resting body temperature. 
• Alcohol: Consuming a few glasses of beer or wine can also elevate body temperature. 
• Sickness: Being unwell can cause a fever, which in turn affects the body’s resting temperature [2].
• Certain conditions: Conditions such as hypothyroidism can affect temperature regulation [5], and those going through perimenopause may also experience fluctuations in basal body temperature due to more unstable hormone levels [6]. 
• Some medications: Check with your doctor if you regularly take medication to find out if it could affect your BBT [2]. 

When using Natural Cycles, you can choose to exclude your temperature if needed so that it’s not taken into account when calculating your fertility status. The NC° algorithm is also able to analyze changes in temperature and can exclude a singular temperature if it’s unusually high or low. The app’s effectiveness remains the same even if your temperature varies from the specific conditions mentioned above. If you’re using NC° Birth Control to prevent pregnancy, the app will give you more Red Days (when you need to use protection) around your fertile window if it struggles to find your ovulation due to varying temperatures. 

How to use the basal body temperature method

Here is what you can expect when using the basal body temperature method:

• Measure your basal body temperature. Use a basal thermometer with two decimal places to measure your BBT. Take your temperature first thing in the morning, as you need to capture your body’s lowest resting temperature as soon as you wake up.
  

  If you're using Natural Cycles to prevent or plan pregnancy, you can also use one of our integrated wearable devices: Oura Ring or Apple Watch. Wearable devices work a bit differently from a BBT thermometer — they don’t measure BBT but rather collect temperature and heart rate data continually throughout the night — meaning that you don’t need to measure in the morning. Whether you choose to measure with a BBT thermometer or a wearable, the NC° algorithm uses your unique data to determine your fertile days. At Natural Cycles, we recommend measuring at least five days a week. This helps give the algorithm plenty of data to narrow down your fertile window and give you more Green (not fertile) Days. Why not check out if Natural Cycles could be right for you?

• Consistency is key. Measure your temperature with the same thermometer around the same time every day to ensure an accurate reading (alternating between measuring devices may cause inconsistencies). Get several hours of sleep each night for the best results. If you have a bad night’s sleep or wake up much earlier or later than usual, you should skip measuring the next day.
  
  
• Chart your basal body temperature throughout your cycles. 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. Your temperature will increase by roughly 0.5-1°F after ovulation [2]. Precision is useful if you’re looking for a hormone-free way to prevent pregnancy or want to find the days when you have the best chance of conceiving if you’re trying for a baby. 
  
  
• Time when to have sex. Since the fertile window is six days long (the egg cell can live for 24 hours, and sperm can live up to five days), you should plan unprotected sex carefully, depending on whether you want to avoid or plan pregnancy. If you’re using the BBT method to prevent pregnancy, you should use protection (for example, a condom) from the start of your cycle until you’ve had at least three consecutive high temperatures indicating ovulation has happened [1].

What should my basal body temperature be?

During your cycle, your basal body temperature will follow a curve. You can expect to have lower temperatures from the first day of your period (which is when your cycle starts) until you ovulate. After ovulation, your temperature will rise and then stay elevated for the whole luteal phase until it drops again just before your next period starts [7].

Before ovulation, in the follicular phase, you can expect your temperature to be around 97 to 98°F (36.1 to 36.7°C). After ovulation, in the luteal phase, you can expect elevated temperatures that are roughly 0.5-1°F (0.3-0.5°C) higher than they were before your ovulation [2].

Now, exactly what temperature you’ll have during these two cycle phases will be individual — each person has their own average temperature range for each cycle phase. It’s also good to keep in mind that your temperature will naturally go up and down a little from day to day within each cycle phase. 

Basal body temperature in early pregnancy

The BBT method can be used to detect pregnancy. One of the early signs of pregnancy is a consistently high basal body temperature continuing after the date that your period was supposed to start [8]. 

Our basal temperature typically dips right before our period during the menstrual cycle. However, in early pregnancy, basal body temperature remains elevated. This is due to the higher levels of progesterone, which stops the uterine wall from shedding, helps with implantation of a fertilized egg cell, and supports an embryo in early pregnancy [9]. 

If you think you might be pregnant, you should always take a pregnancy test or see a healthcare provider to confirm pregnancy. 

How do I know if I'm ovulating with the BBT method?

If you’re tracking your BBT, you will be able to see if you have ovulated. However, it’s important to keep in mind that you’ll only be able to do so after your ovulation has already happened since your temperature will only increase after ovulation. Different versions of the temperature method may have slightly different guidelines, but usually, you can consider that you’ve ovulated after three consecutive high temperatures [1].

You can also track other symptoms, like cervical mucus or cervix positioning, during your cycles along with tracking temperatures to help identify when you’re ovulating. This combination is called “the symptothermal method” [10]. For example, cervical mucus will typically get a color and consistency similar to raw egg whites during the fertile window, so tracking it can help you understand where you are in your cycle.

How the temperature method can help you predict peak fertility and get pregnant

You are at peak fertility in the days just before ovulation and on ovulation day itself [3]. In terms of temperature, this means the best time to try to get pregnant is before your temperature rises. Regularly tracking your BBT can help you find your most fertile days, so if you’re trying to start a family, it’s an affordable and natural way that you can use to work out the best days to conceive. 

However, it’s important to keep in mind that the temperature method only confirms ovulation after it’s already happened — it doesn’t predict your fertile days in advance. That’s where Natural Cycles can help: when using NC° Plan Pregnancy, our temperature-based algorithm helps you more easily predict your ovulation and get pregnant faster by showing you which days you’re most likely to be fertile. 

What are the safe days to have sex using the basal body temperature method?

You can also use the basal body temperature method to prevent pregnancy. Keep in mind sperm can live for up to five days in the female reproductive tract. If you’re using the temperature method to avoid getting pregnant, it’s important to plan sex carefully. 

Because this method doesn’t predict ovulation in advance — and you’re fertile before ovulation — it’s recommended that you use protection from the start of your period until you have ovulated (after three high temperatures) [1]. 

Pros and cons of using the basal body temperature method

Just like any birth control, the basal body temperature method comes with its own pros and cons. Let’s start with some of the pros:

• It’s inexpensive
• You learn about your body and menstrual cycle
• It has no side effects
• You don’t need a prescription

However, this method isn’t for everyone — let’s move on to the cons:

• It’s less effective than many other birth control options
• It requires dedication and effort to measure and track your temperature every day
• A monitoring period (usually three cycles) is needed before you can rely on it 
• You need to be willing to use protection or abstain on days you may be fertile 
• It’s not recommended for those with irregular cycles or certain conditions (like PCOS)
• It doesn’t protect against STIs

When it comes to choosing birth control, you know best the kind of method that will suit you and your lifestyle. If you’re considering this method, take some time to weigh the pros and cons against each other and take your own circumstances into account. Always talk to your healthcare provider before switching birth control to discuss your options.

Natural Cycles is backed by science

While there are many apps available for tracking your cycle and fertility, Natural Cycles is the first and only FDA Cleared birth control app. The NC° algorithm analyzes hormone-driven temperature changes during your menstrual cycle to identify your fertile and non-fertile days, allowing you to prevent or plan pregnancy naturally. Not only that — peer-reviewed research shows that Natural Cycles finds fertile days more accurately than calendar-based methods [11].

Natural Cycles is 93% effective with typical use and 98% effective with perfect use, and it’s just as effective if you exclude a temperature, forget to measure, or if you have irregular cycles. 

Ready to take the guesswork out? 

Fertility awareness methods have been around for decades, but what we do is different. With Natural Cycles, there’s no training or monitoring phase required. Here’s how it works: measure your temperature using one of our compatible devices, log your temperature in the app, add period dates and optional ovulation tests, and the NC° app calculates your daily fertility status for you. Plus, it gives you the option to input and track cycle symptoms you may experience so that you can learn more about your unique cycle. Save yourself the time and effort of counting calendar days and charting yourself — why not give Natural Cycles a try?