Basal Thermometers: What You Need to Know

There are three types of thermometer that you can use to chart your temperature in order to work out when you are at your most fertile:

Liquid Thermometers, also known as spirit thermometers
Digital Thermometers
Infrared Thermometers that are either non-invasive or non-contact

Providing they are accurate to a tenth of a degree centigrade or more (to one point after the decimal point or greater. i.e. 36.7ºc) it doesn’t matter which one you pick. However, there are pros and cons to each thermometer type which you may want to take into consideration when selecting which is best for you.

Liquid Thermometers

+ They are the cheap
+ They are reasonable accurate
– They are not always easy to read
– They can easily break if knocked, dropped or bitten

Digital Thermometers

+ They are usually more accurate
+ Some store previous temperatures
+ Some chart your cycle for you
+ The display is usually easy to read
– They can be expensive
– They need batteries

Infrared Thermometers

+ They instantly take readings
+ You don’t have to put them in your mouth
+ The display is usually easy to read
+ Most allow you to test the temperature of objects
– Some are not very accurate and do not make good fertility thermometers
– These are usually the most expensive type of thermometer
– They need batteries and some drain power very fast

What Do I Need to Know About Liquid Thermometers?

Liquid, or as they are sometimes called “spirit” thermometers, work by inserting them in your mouth, under your tongue for around 3 minutes. You can then read your temperature from the scale they have on their side.

They are by far the cheapest thermometer, but they don’t come with all the frills that some digital thermometers have. However, if your temperature is all you need, this is not a problem.

Some liquid thermometers have very small scales to read and it can be difficult to see exactly the temperature on the thermometer. It is also easy to misread the scale, especially if you are not a morning person (like me) as you take the reading first thing when you wake up.

Old liquid thermometers used the metal mercury, which is now known to be highly poisonous. You should never ever use this type of thermometer and if you have one you should contact your local community household hazardous waste collection service to learn how to dispose of it.

What Do I Need to Know About Digital Thermometers?

You use digital thermometers like you use liquid thermometers by inserting them in your mouth, under your tongue. However, not all digital thermometers need to be kept in the mouth for 3 minutes. But beware some digital thermometers that claim to be high speed use an algorithm (a calculation) to predict what your temperature which can prove to be inaccurate. You should always check the reviews of these thermometers to make sure other people have not found them to be inaccurate.

One of the big plus points for digital thermometers is that almost all of them have easy to read displays. This is a huge advantage over the liquid ones which I have seen many a nurse or doctor misread from time to time by getting the scale wrong on them.

Another plus point for some digital thermometers is that some can remember their previous readings. This is great if like me, you take your temperature first thing, then fall back to sleep. When you wake again, you can turn the thermometer on and it will remember you last reading so you can note it down then instead of when you are sleepy.

Some of the more advanced, fertility specific, thermometers can even chart your fertility for you. This saves time having to draw out separate charts, but be warned some lose their readings when their battery runs out.

On the downside some of the digital thermometers out there can go through batteries very quickly. And it is also worth knowing that some digital thermometers can give inaccurate reading when their battery is coming to the end of its life.

Also cheap is not necessarily good with digital thermometers, as some of the cheap ones are not particularly well made and don’t tend to last long.

What Do I Need to Know About Infrared Thermometers?

Let’s get straight to the point, I do not recommend infrared thermometers for charting your fertility because in my experience most of them are not accurate to a tenth of a degree. These include in-the-ear thermometers. But don’t let me put you off them full stop.

Because infrared thermometers give instant readings and are non-invasive (you can take temperature readings from your ear or even from a couple of inches away from your forehead), they are great for when you do have a baby.

Some even allow you to take the temperature of objects which is great for seeing how hot a bottle of milk or a bath is (providing you have thoroughly mixed them and they have no hot or cold spots).

What Can Make Basal Thermometers Readings Inaccurate?

There are various things which can affect your body temperature. These include:

If you are ill, especially if there is a fever associated with the illness
Drinking alcohol the evening before taking your basal body temperature can cause your body temperature to rise, thus making the morning after readings possibly inaccurate
The surrounding environment also has an impact upon your body temperature, so if there is a change in the normal room temperature, or you are using an electric blanket, or cuddling up more to your partner, this may also make your readings fluctuate inaccurate
Your body temperature will also change on waking up and movements, so if you have had a disturbed or restless night, or needed to get up to go to the toilet within 3 hours of the time of your usual temperature measurement, then the values may also be affected
Some medications may also affect your body temperature, including the use of progestogens. If you are using medication, talk to your doctor about your wish to produce a fertility chart so that they can advise you if your medication would affect this. If your medication affects your basal body temperature, you can still do fertility charting, but use the other fertility signs such as cervical mucus and cervix positioning, rather than temperature monitoring. NEVER stop taking prescription medication without first discussing it with your doctor!

If these situations occur it is still worth measuring your temperature as you would usually do. However, when recording on your fertility chart, it is advisable to either state these disturbances, or mark the reading as questionable so that you are not placing too much importance upon that reading.