Basal body temperature

What is meant by basal body temperature?

In the monthly cycle, there are many different ovulation symptoms that are linked to the hormonal changes within a menstrual cycle. One of the most meaningful is measuring the basal body temperature with a basal thermometer. Analysing these temperature readings in a basal body temperature curve, as shown in the example chart below, is known as the temperature method. It is the basis of natural family planning (NFP).

Example cycle curve: “normal” basal body temperature

Measuring the basal body temperature: what to look out for?

This is how to measure your basal body temperature correctly:

1. Buy a suitable basal thermometer that is accurate to two decimal places.
2. Measure your basal body temperature every day, right after waking up and before getting up.
3. Place the thermometer at your preferred measuring site: vaginally, rectally or orally.
4. Wait for the signal at the end of the measurement so that your temperature is measured completely.
5. Do not change the measuring site within a cycle.

What do I need to measure the basal body temperature correctly?

To record your basal body temperature, you need either a basal thermometer or a cycle computer with an integrated sensor.⁴

A thermometer for the basal body temperature is usually digital and has two decimal places. An analogue version can also be used.

An ordinary clinical thermometer is not suitable, as only thermometers with two decimal places have the accuracy needed to measure the small temperature fluctuations in a woman’s cycle.

You can either enter the readings manually with a pen in a cycle sheet and analyse them, or you can analyse your cycle with the help of a cycle app such as the cyclotest mySense app or a contraceptive computer such as cyclotest myWay.

The cyclotest mySense basal thermometer transmits the temperature readings via Bluetooth to the accompanying cyclotest mySense app. Measuring and analysing the basal body temperature according to NFP is easy and convenient this way. Thanks to the latest measuring technology and the large sensor tip, the digital thermometer has a short measuring time of just 30-60 seconds.

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According to the rules of natural family planning, the standard measuring time is 3 minutes. The measuring time of 3 minutes dates back to a time when thermometers were still filled with mercury as the measuring liquid. With digital measurement becoming established, new measuring times emerged, which are often under 3 minutes.

Our all-in-one device with integrated basal thermometer, cyclotest myWay, also measures the temperature to an accuracy of two decimal places.

When do I measure the basal body temperature?

Here is what you should keep in mind for the perfect time to measure your basal body temperature:

• Measure right after waking up, while lying down.
• Measure before you become physically active.
• Sleep for at least 5 hours.
• Do not get up during the last hour before measuring.⁴
• Measure at the same time each day if possible, in order to detect a reliable rise in temperature.

For your daily basal body temperature measurement, cyclotest mySense offers you a 4-hour measuring window. This ensures that you get comparable readings.

Where is the best place to measure the basal body temperature?

The basal body temperature can be measured orally, rectally or vaginally:

1. Oral measurement: Place the measuring tip under the tongue, right next to the frenulum, in one of the two heat pockets. Keep your lips closed, as an open mouth can cause the oral cavity to cool down and distort the temperature reading.
2. Vaginal measurement: Insert the measuring tip about 2 cm into the vagina. You can do this during your period as well.
3. Rectal measurement: Insert the measuring tip about 2 cm into the anus.

What can disturb the basal body temperature?

There are certain disruptive factors that can influence the basal body temperature:

• Medication
• Illnesses, such as a fever or a cold
• Alcohol consumption
• Stress
• Sleep disorders or too little sleep

In such cases, it can be advisable to skip measuring your basal body temperature. This is known as “excluding” a reading. If you only realise afterwards that your temperature was disturbed, you can record the temperature reading of the current cycle day as a disturbance in the cyclotest mySense app. This way, the disruptive factor is not included in the analysis of your infertile and fertile days.

Alcohol as a disruptive factor can influence the basal body temperature

Irregularities of this kind can affect the temperature, but they do not have to. This differs from woman to woman. When in doubt, you can exclude the measured temperature reading if it deviates too much from the other readings. Measuring and analysing the basal body temperature with the aim of drawing conclusions about your own fertility is known as the temperature method.

When is the basal body temperature elevated and when is it “normal”?

In the first half of the cycle, that is from menstruation until ovulation, the normal basal body temperature is around 36.5 degrees Celsius. The corpus luteum hormone progesterone, a progestogen, is produced in the corpus luteum in the second phase of the cycle after ovulation. Progesterone now further prepares the uterine lining for the implantation of the fertilised egg. Progesterone has the property of acting on the body’s heat regulation centre and raising the temperature. Since this sex hormone is only produced when ovulation has taken place beforehand, an elevated temperature can be regarded as a reliable sign of ovulation.¹

What is the basal body temperature like during ovulation?

Within an undisturbed cycle, there are two temperature levels:

1. Low phase: In the first half of the cycle, the basal body temperature is somewhat lower, which is why this phase is also called the low phase. 1 to 2 days before the rise in temperature, you can often notice a slight drop in temperature. This temperature low occurs shortly before ovulation.
2. High phase: The waking temperature then rises by at least 0.2 degrees Celsius in the second half of the cycle. This second phase, which lasts until the next menstruation, is called the high phase.²

The World Health Organization (WHO) has defined criteria for inferring ovulation from the basal body temperature:³

If the temperature level remains equally low throughout the entire menstrual cycle (monophasic), ovulation has probably not taken place.

What is the waking temperature like before the period?

If the egg was not fertilised, the corpus luteum dies 12–16 days after ovulation and the progesterone level decreases. As a result, the temperature drops again shortly before the start of menstruation.

What is the basal body temperature like when you are pregnant?

If you keep diligently measuring after the high temperature phase has set in, you can use your cycle curve to determine whether you have managed to get pregnant. If more than 18 elevated readings are displayed, this is a reliable sign of pregnancy.