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What is Basal Body Temperature?

If you’re trying to get pregnant, understanding your ovulation cycle can greatly increase your chance of conception. While you will have to use a fertility monitor or understand and chart changes in cervical mucus or luteinising hormones (which can be determined with an ovulation test) to predict when you will ovulate before it occurs, you can verify whether or not you are ovulating by charting your Basal Body Temperature (BBT).

So, what is Basal Body Temperature?

Your Basal Body Temperature is simply the lowest temperature your body reaches while you sleep. The significance this holds to your ovulation is that when ovulation occurs, your core body temperature rises about half a degree and stays elevated for about 1 week. By consistently monitoring your BBT you can effectively determine whether or not you are ovulating simply by the natural shift in body temperature.

Checking your Basal Body Temperature and understanding the changes that occurs during ovulation is crucial to increasing your chances of pregnancy, but is quite simple once you know what you’re doing.

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7 things you need to know about checking your BBT:

  1. Purchase a thermometer specifically designed for checking your basal body temperature. These thermometers will be extremely precise and need to show at least to the tenth degree. Start recording your temperature during the first day of your menstrual cycle.
  2. Whether you decide to check your temperature rectally, vaginally, or orally make sure you check it the same way every time with the same thermometer.
  3. It’s important that you check your BBT immediately after waking. Do not make any physical movements; don’t eat or drink anything; do not smoke before checking your temperature; don’t even get out of bed.
  4. Always take your BBT around the same time every morning—there is a 30-60 minute window that’s allowed, but try to keep it as close to the same time as possible.
  5. Make sure you get enough rest; your body needs time to enter a deep, restful sleep so that your temperature can reach its lowest point. Plan to rest at least 4 hours before taking your BBT.
  6. Don’t make any changes to your temperature. You may be tempted to raise or lower it because you took it too early or too late but listing it exactly as it was is important. It’s also important to note that drinking alcohol or getting too little sleep can have an impact on your BBT, so avoiding alcohol before bed and getting as much rest is important.
  7. Always write down your results. There’s usually a small chart that comes with your thermometer for recording temperatures; don’t rely on your memory as you could forget or get mixed up from one day to the next.

Establish Your Coverline

To determine if you are ovulating you will need to establish your coverline. The coverline is simply a line that will allow you to represent the time of ovulation. If you see a spike in your temperature that is more than two-tenths of a degree and lasts at least two days, you can begin your coverline—you are most likely ovulating during this period. Once your temperature goes back down to what it was during our menstrual cycle, you are no longer ovulating.

Buying A BBT Thermometer

If you want to start charting your Basal Body Temperature on a daily basis, you will need to invest in a suitable thermometer and a supply of fertility charts on which you can record the results of your daily readings.

It doe not really matter whether you record your temperature in Centigrade or Fahrenheit. This choice is purely down to your personal preference.  At Zoom Baby we offer both types of thermometer, at the same price, so you can choose the one you feel most comfortable in using.

°F (Fahrenheit) BBT Thermometer

Our Fahrenheit thermometer is ideal for charting your BBT. Just before ovulation occurs, your body will encounter a slight temperature drop. This is followed by a distinct temperature rise of about 0.40 to 1.0 degrees Fahrenheit after ovulation takes place. By plotting your temperature change during your cycle you will know when you have ovulated.

The Zoom Baby Fahrenheit Thermometer comes with full instructions on how to use it. You also receive a free, printed fertility chart which can also be downloaded for future use.

°C (Centigrade) BBT Thermometer

Our Centigrade thermometer is ideal for charting your BBT. If you have a regular, 28 day menstrual cycle, you body temperature will normally hover around 36.5 degrees Celsius. Just before ovulation takes place, your temperature is likely to drop to around 36.2 degrees. Normally this drop occurs at around day 13-14 of your cycle. After this, when you ovulate, your body temperature will increase and continue to climb until it peaks at around 37 degrees. It will stay at this temperature until just before you have your period.

The Zoom Baby Centigrade Thermometer is supplied with full instructions on how to use it. You also receive a free, printed fertility chart which can also be downloaded for future use.


This post has been updated since its original publication in 2013.
Photo by Yohann LIBOT on Unsplash
Zoom Baby is a leading supplier of Pregnancy Tests and Ovulation Test Kits


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