Basal body temperature: What it is, how to chart it, and more

If you’re interested in monitoring your fertile days, chances are you’ve heard of the basal body temperature (BBT) method. This technique is a popular method as it’s cheap and easy to do at home with a thermometer. It’s not, however, a perfect method to use as it only tells you after you have released an egg which means your fertility window has closed for that menstrual cycle. But what exactly is basal body temperature, how does it work, and how effective is measuring it in helping you track your fertile days? Read on to have your questions answered about this natural family planning method.


What is basal body temperature?

Your basal body temperature (BBT) is the temperature of your body when you’re fully at rest. 

Tracking BBT is a tool some women use to understand the pattern of their fertile days in each cycle, by detecting a slight rise in their BBT after they ovulate. 

However, if you’re trying to conceive it can be quite inaccurate, since you are at your most fertile only in the days leading up to ovulation and the day of ovulation itself, so by the time you notice a rise in temperature, you’ll have missed your fertile window

How can BBT indicate ovulation?

When you ovulate, there are physiological changes that come with it. One of these changes is an increase in BBT of around 0.2-0.6°C, for at least three days. Often, it’ll stay high until your next period starts and a new menstrual cycle begins. 

Measuring BBT only tells you when you’ve already ovulated, and since the egg only survives for up to 24 hours, by the time you notice a rise in temperature, it’s too late to conceive. 

If you’re trying for a baby, it’s not just the day of ovulation that’s important, but the days leading up to ovulation (your fertile window). Since sperm can survive for up to five days, the days before you notice a change in your BBT are high fertility days when you have an increased chance of getting pregnant. 

This is why the BBT method can be quite inaccurate if you’re looking to conceive, in fact studies have shown that it is only up to 70% successful in predicting fertile days (to within one day)1. If you do decide to use it, it will help you understand your cycle, and you could use it together with another method like the cervical mucus method or to build up a pattern. 

If you’re looking to accurately identify your most fertile days, using an ovulation test will give you at least 99% accuracy at detecting the LH surge which occurs before ovulation. Some ovulation tests also detect estrogen, to identify a wider fertility window—as estrogen increases before LH does—and can help increase your chances of getting pregnant.  

How to take your basal body temperature and keep track of it?

  • Take your BBT every morning before getting out of bed. You can use a digital thermometer you keep by your bed or one that’s specifically designed for measuring BBT. 
  • Try to make sure you’ve had three hours of uninterrupted sleep beforehand. The results are more likely to be accurate if you measure your temperature after a longer period of uninterrupted sleep. 
  • You can take your temperature orally, vaginally, or rectally. Use the same method each time for consistent results.
  • Try to take your temperature at the same time each day. It’s best to try to take your temperature as close to the same time each day as possible. Staying within a 30 minutes margin on either side of your average time is still fine. You might want to use an alarm as a reminder. The timing is important because your BBT can vary up to 0.1 degrees per hour. 
  • Record your temperature on a daily chart. Make a note of the temperature on the chart each day, but don’t read too much into it until the cycle is over and you can look back to see a pattern. 

What is a BBT chart and how do you interpret it?

A basal body temperature chart is a record of the temperature measurements you’ve made every morning over a menstrual cycle. Looking back at it, you will be able to see when there is a change in BBT, which indicates when you ovulated that cycle. Having this data can be a little confusing, so here are some things to consider when charting your BBT:

  • A temperature change, usually a rise of around 0.2°C, that remains steady for three days or more indicates ovulation has happened. 
  • A single temperature shift on the chart can be discounted as not being linked to ovulation and could be due to something else, for example an illness or external cause. 
  • Analyse the chart at the end of each cycle. 
  • Record your BBT over a few cycles to see any patterns in your temperature. 
  • If you notice your temperature remains high for 18 days or more after ovulation, and your period hasn’t started you may want to take a pregnancy test. A continuous high BBT after ovulation can be an indicator of early pregnancy. 

 

 

What are the advantages of the basal body temperature method?

The advantages of using the BBT method are: 

  • It’s cheap. 
  • It’s easy to do at home. 
  • It can help you understand your menstrual cycle and your fertility. If you’re trying to get pregnant, your BBT chart can help you confirm ovulation.    
  • There are no health risks or side effects. 
  • It is a method to confirm ovulation has occurred.

What are the disadvantages of the basal body temperature method?

As mentioned earlier, the BBT method can be inaccurate1.

There are also a few factors that can give you misleading results, for example 

  • illness 
  • changes in sleep patterns 
  • alcohol consumption 
  • shift work
  • interrupted sleep cycles 
  • travel
  • certain medications. 

Another disadvantage is that BBT won’t tell you when you are entering your fertile window—it’ll only tell you once you’ve ovulated by which point your fertile window has already closed. This means the method only really works well if you have regular cycles that you can track for a few months until you can see a pattern emerging.

Can you use basal body temperature to tell you when you’re pregnant?

If your BBT rises and stays up for 18 days or more, it may be an early indicator of pregnancy. Should you notice your BBT has risen and isn’t going back down, you may want to take a pregnancy test. Whilst the rise in BBT maybe an indication it’s a good idea to confirm by doing a pregnancy test. 

If you don’t want to wait until your missed period to confirm you are pregnant, you can take a pregnancy test as early as 6 days before your missed period, which is 5 days before the day you would expect your period to start. With the Clearblue Ultra Early Pregnancy Test 79% of pregnant results can be detected 6 days before your missed period

When should you see a doctor?

Although there are no health risks associated with measuring your BBT, your pre-natal health is important for a baby’s health so we recommend you see your doctor first before trying to get pregnant.

FAQs about basal body temperature

The BBT method can be a useful way to understand your own fertility, but when it comes to trying for a baby, it’s probably worth considering another fertility tool. If you’re looking to get pregnant and want to accurately determine your fertility window, the most accurate home method is to track your key fertility hormones. You can do this using an ovulation test.

  1. Brezina, P. R., Haberl, E., & Wallach, E. (2011). At home testing: optimizing management for the infertility physician. Fertility and Sterility, 95(6), 1867-1878