Basal temperature and ovulation: how to identify fertile days?

Step by step explanation to measure basal body temperature correctly to monitor your menstrual cycle and understand when ovulation occurs.

Basal temperature measurement is a method to be able to identify ovulation and fertile days , especially for those who have difficulty assessing cervical mucus or have an irregular cycle.

In this article, we will explain in detail how this method works and what information it allows to obtain.

In addition, we will give you all the tools to create your basal temperature graphs and to interpret them correctly.

What is Basal Temperature?

The basal temperature or TB is the temperature of our body recorded in basal conditions, i.e. upon awakening and when the body is still in total rest, before starting any activity (even minimal).

To identify ovulation and fertile days, the temperature must be measured every day and recorded on a special graph , which you must then learn to interpret

What is the relationship between basal body temperature and ovulation?

The basal temperature method is based on the fact that the basal temperature changes after ovulation .

In fact, once the woman has ovulated, progesterone causes a rise in body temperature of about 0.2-0.5 degrees Celsius and this rise lasts until the progesterone level drops at the beginning of the menstrual cycle, due to of the degeneration of the corpus luteum .

If your basal body temperature remains high  and your period doesn’t come, it could be a sign that you are pregnant .

When and how is basal body temperature measured?

Body temperature should be measured every day (possibly skipping menstrual flow days), immediately after waking up and before getting up and doing ANY activity .

Also, you need to sleep at least 3 consecutive hours before detecting it.

It is preferable to measure the basal body temperature before 8 in the morning . Late morning measurements may be skewed.

The time of measurement must always be the same , with a difference of up to an hour and a half between the different days .

That is why those who work shifts or suffer from insomnia cannot use this method. If you use it can get uncertain results.

Remember to always indicate the time of measurement in the graph , it is a very important parameter that can explain the anomalies that are obtained in the graphs.

In addition to the time, it is essential to always adopt the same measurement method , always trying to recreate the same conditions and using the same thermometer .

If you have an illness or other condition that could alter your temperature readings (for example, if you went to bed very late or had a larger dinner than usual), remember to mark it on the graph.

Method and thermometer to use

Choose a temperature detection method (preferably vaginal or rectal, no ear or axillary) and always use only that.

There are precise indications on how to measure the basal temperature and for this reason we recommend that you read the following in depth to avoid making a mistake:

You can use your usual thermometer. However, if you prefer to have a personal thermometer to always keep on your bedside table, there are different types on the market.

For ease of reading it is better to choose a digital thermometer with one or – even better – two decimal places. In pharmacies there are special thermometers to measure the basal temperature.

The cost varies based on the type of thermometer you choose. The more expensive ones store up to 60 days of readings, others are associated with apps, such as the Femometer thermometers .

What information can the basal temperature give?

Basal temperature and ovulation

Basal temperature measurement is the only home method that allows you to confirm ovulation , thanks to the rise in temperature caused by progesterone.

Of course, ovulation can also be confirmed by ultrasound or blood progesterone testing, but these methods require medical intervention. The basal temperature, on the other hand, can be measured independently.

Obviously, when you observe the rise in basal temperature, ovulation has already occurred .

This method, therefore, does not serve to predict the fertile period of the current cycle , because the fertile days are those immediately preceding ovulation, but to understand when ovulation occurs and therefore to be informed for the next cycle.

Basal temperature and pregnancy

A basal temperature that remains high for at least 16 consecutive days after ovulation can be an indicator of pregnancy .

The post-ovulation rise can be marked or less evident, as it is possible to see from the pregnancy graphs  that we have collected in a dedicated article.

More information beyond ovulation

Basal temperature could signal some physiological imbalance. For example, if it is always below 36°C during the follicular phase , it could indicate a slow metabolism possibly associated with hypothyroidism . Symptoms in this case are lethargy, weight gain and sensitivity to cold.

On the other hand, a high temperature before ovulation could suggest hyperthyroidism .

If the temperature remains constant all the time , it could instead indicate that there has not been ovulation , which can happen after discontinuation of the pill or in the presence of luteal phase defects.

In the latter case, if the woman is trying to get pregnant, the doctor may suggest that she resort to therapies such as ovarian stimulation and the administration of progesterone .

Is basal body temperature reliable?

The basal temperature method is not very reliable because, as you can imagine, the body temperature can increase for various reasons such as viral infections, stress, drug action, alcohol intake, travel, time difference or simply just from going to bed or having had a late dinner  the night before.

It happens very often to see altered temperatures on the weekends in the graphs . This is because on Fridays or Saturdays people stay up late or eat late, more or differently.

All of this can have an effect on the basal body temperature taken the following morning.

Typical trend of a graph

Basal temperature before and after ovulation: the biphasic graph

After the menstrual flow, the basal body temperature is quite stable.

The decrease before ovulation (caused by the increase in circulating estrogen) may not be evident, on the contrary there may be a very slight rise in temperature (about 0.1°C) on the day of ovulation , due to progesterone which is produced just before the follicle bursts.

The biphasic trend of the graph between before and after ovulation must be recognizable (see graph below). Ovulation usually occurs the day immediately before the temperature rise.

To confirm ovulation, the rise must persist for at least 3 consecutive days .

The sudden drop in temperature 11-12 days after ovulation indicates that menstruation is about to arrive.

atypical variations

In some cases you may notice a progressive rise in temperature and therefore not a clear step. However, ovulation takes place the day before the rise begins.

Sometimes, 5-7 days after ovulation there may be a sharp drop in temperature but it rises again the next day . This can be a sign of implantation (implantation of the embryo in the uterus) and is called ‘implantation loss’.

What if ovulation doesn’t happen?

Logging tool to track your own fertility

We have prepared for our readers a free tool for recording basal temperature and symptoms ( cervical mucus , sensations, etc.), here on the site. It is a personal and private tool , to keep your data monitored on a daily basis.

The data must be entered every day starting from the first day of menstruation, which corresponds to the start date of the menstrual cycle. In fact, we remind you that, from a medical point of view, the menstrual cycle does not correspond to the days of the flow but to the period that passes from one period to the next (which on average is 28 days).

Every day, enter the temperature measured, any symptoms, intercourse, etc. in the graph.

I recommend doing this chart even to those who are not trying to get pregnant. It is a great way to understand how our bodies work. Even if you don’t want to have a baby, it’s a great workout to figure out which days are fertile during the period between one period and the next.

Do you prefer pen and paper? You can use the paper chart

Download the guide sheet in PDF format and print it. You need one for each menstrual cycle during which you want to check your basal body temperature. Always keep complete copies, they may be useful to your gynecologist.

  CLICK HERE TO DOWNLOAD AND FILL IN THE BASAL TEMPERATURE CHART (PDF) (43617 downloads)

Here is a sample basal body temperature chart (click image to enlarge). I compiled it directly with Excel, so if you prefer to edit the Excel file from month to month instead of filling in the printed PDF by hand, here is the link to download it (it can also be opened with Open Office):