This article describes how to measure basal body temperature (BBT). It gives instructions on taking BBT measurements rectally, orally and vaginally. What are the features of each method? And also how to choose the best measurement method.
The oral method of measuring basal body temperature – the thermometer is placed under the tongue for about 8 minutes (digital thermometer until sound signal). With the rectal and vaginal methods, the thermometer is immersed 2-4cm deep for about 5 minutes (digital thermometer until sound signal). The most accurate method is rectal and vaginal. The most popular is for the measurement to be taken orally.
Where is the best place for measuring basal body temperature?
Let me remind you of the basic principle of evaluating and decoding the basal body temperature chart: it is not the absolute indicator (for example, 36.7 °C, 36.8 °C, 37 °C) that matters, but the difference, the deviation between them. For example, the indicators show 36.6°C (97.88 °F ) in the first phase of the cycle (before ovulation) and 37.0 °C (98.60 °F) in the second (after ovulation) – the values themselves aren’t considered indicative, what matters (!) is the temperature difference between the average values of the two phases, which is 0.3-0.5 °C (0.54–0.9 °F).
Note(!): To obtain the most accurate results, that are not distorted by external factors, you need to adhere to a few simple measurement rules. Among them: use the same thermometer, measure at the same time, sleep 3-6 hours of sound sleep until this moment, and others.
Measurements taken orally
If the basal body temperature is measured orally, you shouldn’t do unnecessary gestures or any talking. After waking up, you should quickly put the mercury thermometer under the tongue and wait for about 8 minutes. If an electronic thermometer is used, you need to wait until the sound signal.
Important(!): The thermometer should not cause pain, pressure or other discomfort. Since this could stimulate a rush of excess blood and, as a result, tissue heating leading to an increased BBT result.
For the oral method of measurement, electronic and glass (mercury or non-mercury) thermometers are equally suitable.
How to measure basal body temperature vaginally (least popular method)
For proper measurement, you need the following:
- Upon waking with as little movement as possible, lie to one side;
- Place a pre-prepared thermometer approximately 3-4 cm into your vagina;
- After about 5 minutes, take the thermometer out and evaluate the result.
As in the first case, there should be no discomfort and pain after immersion of the thermometer.
Important(!): When measuring with a mercury thermometer, it is necessary to evaluate the results immediately, since the mercury column may drop by 0.05 °C within a short time. Read more about the advantages and disadvantages of various BBT measurement instruments in this material.
The vaginal method is the least popular among women, since it carries a risk of infection and injury. If you are a beginner, it is better to choose an oral or rectal method of measurement (I did, when tracking ovulation for both of my pregnancies).
Basal body temperature measured rectally
This method is good because rectal BBT is akin to vaginal (located physiologically close), but at the same time there is less risk of infection. Duration of measurement – about 5 minutes with a glass thermometer.
This is interesting(!): When measuring basal body temperature, each method can give different results at the same time. For example, indicators of oral BBT are almost always lower by 0.2 °C (0.36 °F) than vaginal and rectal BBT indicators. Hence the conclusion: always use the same method and the same thermometer.
If you cannot decide which measurement method to choose, choose the most accurate measurement method – rectal with a cheap mercury thermometer (it’s cheaper than non-mercury and easier to care for than electronic).
Further down with practice, as you gain experience, you can switch to the oral method using a special BBT measuring electronic thermometer. Change the instrument (and/or method) from the first day of the cycle so that the phase difference is more accurate.
Remember, basal body temperature is indicative, regardless of the chosen method. It’s all about the convenience and personal preference, following the provided simple rules for measuring basal temperature and how to calculate ovulation with BBT with examples.