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Why Use Daysy for Monitoring Your Cycle Instead of a Regular Menstruation App?

menstruatie-app op je mobiel niet betrouwbaar

Menstruation apps are fine to use to track your periods but to track your ovulation and fertile days, you need to know exactly how natural family planning works. You cannot rely on what a standard menstrual app indicates about ovulation and fertile days before ovulation. Even if you have an irregular cycle, you want to be able to rely on an accurate reliable calculation of your personal menstrual cycle.

Disadvantages of a menstrual app

There are also a number of disadvantages to using menstrual apps, including a very objectionable one: the invasion of your privacy. Because did you know that most apps make money by passing on your personal data to third parties?

Are you considering using a menstrual app or are you already using one, read on about the (un)reliability of these apps.

Physical signs of ovulation used in menstruation app

First, let's take a look at your body's physical signs of ovulation. Because when you use a natural family planning method, you need these signals from your body to determine your fertile days. Suppose you have no desire to have children then you use a barrier method on these fertile days when you have intercourse.

Basal body temperature

Your basal body temperature (BBT) is the most important clue used in natural family planning. It is the temperature you measure under your tongue in the morning, right after waking up. During your cycle, different hormones affect your basal body temperature. At the beginning of your cycle, your body mainly produces oestrogen and you have a relatively low body temperature. The second phase of your cycle, after your ovulation you will see a change in your temperature curve. Your basal body temperature rises due to the hormone progesterone. So you can recognise your ovulation by a rise in your temperature, your ovulation is before the rise.

basale lichaamstemperatuur meten met medisch apparaat Daysy betrouwbaar

Cervical mucus

During your cycle, the composition of your secretions changes. For instance, in the days before you ovulate, you may clearly notice that the colour and texture of the discharge has changed to clear, stretchy and smooth 'fertile mucus' (cervical mucus). This allows sperm to move faster through your uterus to the egg. After ovulation, the amount of discharge decreases again and becomes opaque again.

It is important to note that interpreting cervical mucus alone is not very reliable. For example, you can have fertile mucus early in the cycle if you have a high oestrogen level compared to your progesterone level. And even after ovulation, you can have fertile mucus if you don't make enough progesterone.

hormonen tijdens je menstruatiecyclus, vaginaal slijmverlies gedurende de cyclus

Location of the cervix

When you feel inside your vagina with one or two fingers, you can feel your cervix. Normally, your cervix lies low (a finger's length into your vagina) and feels hard (similar to the tip of your nose). The days before ovulation, your cervix lies higher, opens slightly and feels soft. After ovulation, the cervix closes again and becomes harder again.


Every month when the ovaries get ready to release an egg, they produce oestrogen. Estrogen, in turn, stimulates the neurotransmitter serotonin, which can make women more cheerful and energetic in the few days before ovulation. Oestrogen also positively affects your libido.

Want to know more about ovulation? Read our detailed blog 'How to recognise ovulation with your Daysy Cycle Tracker'.


As you will understand, in the method of natural family planning, it is very important to correctly track and interpret the physical signs of ovulation. Human errors such as not entering data correctly into a menstruation app have a major impact on the reliability of the method. And as we wrote above: signs like fertile mucus, lie cervix and mood alone are not reliable enough. Measuring basal body temperature is the only way to actually determine whether ovulation has occurred.

Ladies who measure with the Daysy know this. Daysy is a smart algorithm is you can trust. It is a medically certified device that uses a computer algorithm to indicate your fertile and infertile days - based on your basal body temperature - with 99.4% accuracy.

The technology of the internal algorithm in the Daysy has been used worldwide for 35 years and has been scientifically researched. This is unlike menstrual apps.

As you have read above, measuring basal body temperature is the only truly reliable way to determine your fertile and infertile days: your body shows by a rise in your temperature that ovulation has occurred.

But did you know that not even all apps use basal body temperature to determine ovulation? For example, there are apps that just do simple arithmetic: you enter the first day of your last period plus the length of your cycle. Based on that, the app calculates when ovulation is: 14 days before your next period. This way of calculating ovulation is quite outdated as the length of your cycle can vary monthly and ovulation does not always occur on day 14 of the cycle.

Did you know that sperm can survive internally for up to 5 days? Then it's nice that you get those red fertile days from your Daysy. You don't want to risk sperm being able to fertilise the egg on the day of ovulation. These are the important days if you have a desire to have children but if you don't have that desire yet you want to be warned early before ovulation so you can use condoms.

As many as 70% of women ovulate outside the expected range of day 13-15 in the cycle so determining ovulation with this simple math in the menstruation app is not very reliable.
meet met cyclus tracker daysy, meet je hormonen

So measure your BBT every morning as soon as you wake up with Daysy to see whether you are in your fertile period (red) or infertile period (green). The algorithm Daysy uses for this is based on five million menstrual cycles and a smart real-time calculation. Daysy learns from your cycle pattern but adjusts the colours if your cycle is currently different. Suppose your ovulation is slightly later this month, your temperature will not rise enough yet. Daysy will then continue to give red until the progesterone has caused your temperatures to be stably higher than before ovulation. That is when Daysy gives green. This will be different for every woman and can sometimes take a few days, as every woman has a unique course of her cycle. During fertile days, use a barrier method if you don't want to be pregnant.

Now you may be wondering, "I can use a regular thermometer from the drugstore, right?" Then be aware that Daysy has a highly sensitive sensor to precisely measure basal body temperature under the tongue. The most unique feature of this sensor is that it waits for the average temperature reading, which can take up to 90 seconds.

The sensor warms up before registering basal body temperature to get an accurate result. It waits for the temperature value to stabilise to complete the measurement. If the temperature drops during the measurement (for example, because you breathe cold air), the sensor waits until the temperature stabilises again. This unique feature sets the Daysy apart from ordinary thermometers that usually have a fixed measurement time. In which your exact temperature may not yet be reached.

Privacy and protection of your data

Be aware that most menstrual apps use your data for marketing and advertising purposes. For instance, your personal data is passed on to third parties to make money. In addition, many apps are not hack-resistant, so your privacy-sensitive data (ip address, phone number, date of last menstruation, due date) could be made public just like that.

At Daysy we work with the DaysyDay app, by synchronising, the measurements from Daysy can be seen back in the app to gain more insight into your cycle progress. The data (measurements) are only used to display your cycles and statistics and not passed on to third parties.

To transfer the data, it is necessary to use our servers. Because we know how sensitive this data is, it is transferred encrypted and pseudonymised to our servers within the European Union, and stored there.

This means that even if unauthorised third parties were to gain access to the servers, the encryption and pseudonymisation make it impossible to assign your data (basal body temperature, menstruation, ovulation, fertility status) to a specific person. This particular protection is very important to Daysy.


Daysy expensive? Daysy may be a more economical choice because a drawback of menstrual apps is that they often use a paid membership/subscription, sometimes as much as €10.99 per month. If so, Daysy at €299 with a lifetime of (at least) five years is a more economical option (€10.99 per month for five years = €659.40).

Tracking of (needles) information

And finally, the consideration of whether filling in the many pieces of information in apps is necessary. Many menstrual apps advertise the ability to track a wide range of information, for example: weight, amount of water drunk, status of your skin and hair, weight, stools, digestion, social, activities. Be aware that all this information is not necessary for calculating your fertility. And when you feel the 'need' to keep track of everything, it can cause stress which can have a negative effect on your cycle.



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