What Is a Basal Thermometer?

A basal thermometer is an essential element for accurately knowing the temperature of the body and inferring the time of ovulation.
What Is a Basal Thermometer?
Leidy Mora Molina

Reviewed and approved by the nurse Leidy Mora Molina.

Last update: 27 December, 2022

There’s a method of fertility control based on the recording of a woman’s basal temperature throughout her menstrual cycle. To do this, you need to use a basal thermometer, which is a more accurate measuring instrument than those commonly used.

Keep reading and find out more about the usefulness of this tool in family planning. Don’t miss it!

What’s a basal thermometer and what’s it used for?

A basal or ovulation thermometer is a very sensitive instrument that records body temperature more accurately than those in everyday use. Unlike these, its display shows two decimal places instead of one and can be used orally, rectally, or vaginally.

The accuracy threshold of the basal thermometer allows it to detect minimal fluctuations in temperature, which are key data to determine the hormonal changes of the different stages of the menstrual cycle.

 Basal temperature is the temperature that a person maintains at rest, that is, after having remained at least 4 hours without activity. For example, when waking up in the morning, before sitting up, or getting out of bed.

Certainly, this thermometer is the preferred element for women who opt for the contraceptive method based on basal temperature. But in addition to family planning, the tool is used for other purposes, such as monitoring the evolution of thyroid diseases.

A basal thermometer measuring 36.52 degrees celsius.
Basal thermometers determine basal temperature more accurately, as they record two or more tenths of this parameter.

Types of basal thermometers

Basal thermometers are available on the market in two formats: Digital (plastic) or gallium (glass). The former is the most commonly used, as it records the temperature more quickly and also provides accurate values to two or more decimal places. The main advantage of the glass ones is that they’re cheaper.

Why is it important to choose a basal thermometer instead of the common one?

Definitely, if you need to record an accurate basal temperature, you should choose the basal thermometer. This parameter doesn’t usually change by more than a few tenths of a degree during the female cycle and therefore, a common thermometer wouldn’t be able to detect the variations.

While a common thermometer determines the temperature in degrees and with only one decimal place, a basal thermometer does it with two or more decimals, for greater accuracy.

As we’ve anticipated, family planning method based on basal temperature consists of recording the changes in this vital sign in order to predict the probable time of ovulation. That is, the most fertile days of the menstrual cycle.

If the temperature at rest is monitored daily, just before ovulation, the woman will notice an increase of a few tenths of this parameter and thus make the decision to conceive or not to conceive.

After ovulation, the exponential increase of the hormone progesterone will cause the woman’s basal temperature to increase between 0.2 and 0.45 ° C and this will indicate the most fertile time of the cycle.

Based on the recording of this temperature, a graph is made to analyze the characteristics of the menstrual cycle of that person and from here, estimate the days on which you can get pregnant. However, for greater reliability, it’s best to use the symptothermal method, which combines basal temperature, changes in cervical mucus, and the calendar method.

How is basal temperature recorded?

To make the monthly basal temperature graph, you should place the temperature on the vertical axis and the days of the menstrual cycle on the horizontal axis.

Every day, you should take your temperature with the basal thermometer when you wake up, ideally at the same time, without getting out of bed. Keep in mind that it’s best to always use the same route (rectal, oral, or vaginal).

Once the value is obtained, record it on the graph. Near the middle of the cycle, you’ll notice a sharp increase, which will be about .5 °C. As it’s difficult to determine the exact moment when the egg leaves the ovary, consider that your fertile period begins 4 days before ovulation and ends about 24 hours later.

Before implementing the method as your only form of contraception, it is recommended that you try it for at least two menstrual cycles. This is so that you get used to recording your waking temperature and know how to interpret the results of the graph.

A doctor explaying the ovulation cycle to her patient.
Consult your gynecologist before implementing the basal temperature method as a family planning strategy. The professional can teach you how to use it correctly.

If you want more reliable results, use a basal thermometer

A basal thermometer is designed to obtain more accurate and reliable temperature records, especially to contribute to the couple’s family planning.

Remember that there are different formats on the market, so we recommend you look for the one that best suits your needs. The best attribute that a measuring tool should have is that it’s practical, accessible, and allows you to use it correctly.

Before using the basal temperature method, talk to your doctor and ask for his or her advice, especially if you want to use it as a contraceptive.

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.