Fertility Tracker Daysy as Birth Control : Why I Stopped Using It

May 17, 2020 | Menstrual cycle

Want to learn more about the fertility tracker Daysy? Here we go.

But before, please remember that this is my personal opinion. This is not medical advice. Make sure you talk to your doctor before changing your birth control (even if natural!)

Ok, now, let’s go!

Daysy is a fertility monitor that enables women to track their menstrual cycle naturally. After taking the decision of not going back to hormonal birth control, I tested Daysy for a year.

In this article, I share my experience with the fertility tracker Daysy, and why I don’t recommend it as a contraceptive.

If you read my previous posts about birth control, you would have known that I suffered from the pill’s effects after taking it for years.

That’s the reason why I decided to ban hormones from my life and try to find a natural way to monitor my fertility.

Before starting with the fertility awareness method that I practice for almost one year now, I first tried the fertility tracker Daysy.

For your information, I tried the first version of it and therefore did not include all functionalities of the last one.

What is Daysy? 

Daysy is an intelligent thermometer that tracks your cycle through an app.

The monitor supposedly bases itself on “an intelligent algorithm based on scientific research, many years of experience of more than 500,000 users and sophisticated technology” (source).

This way, the fertility tracker Daysy analyzes your temperature every day to determine your cycle’s status. 

After analyzing your basal temperature (that you take first thing in the morning), the monitor shows a green light if you’re in an infertile period, a red light if you’re in a fertile period, and a yellow light if it’s uncertain. 

Daysy is therefore partially based on the traditional Fertility Awareness Method. If you don’t know what I’m talking about, check out my article on the FAM.

Is Daysy reliable?

When trying to know more about its reliability, it gets complicated. When I first bought the monitor in 2018, the website claimed 99.4% reliability, which is pretty high and more or less equal to the pill.

In May 2019, the Journal of Reproductive Health retracted the study on which was based this reliability rate (source).

According to Valley Electronics LLC, the authors didn’t agree with the retraction, but the company still decided to stop using this rate to promote its product.

Today, we don’t have any information in regards to the fertility tracker’s reliability.

My experience

I got myself the fertility tracker Daysy in December 2018 and started using it religiously in January 2019. I used it until January 2020 which makes a whole year of me trying it.

During the first months, the monitor has to learn the body and therefore indicated many red and yellow lights.

It took approximately 6 months for it to show me more infertile days and enable me to have unprotected sex again.

When I found Daysy and started using it, I had no idea how it truly worked, nor what the fertility awareness method was.

But following closely my cycle made me want to learn more about it and to find what was hiding behind the algorithm.

That’s when I sought help and went to a FAM advisor in November 2019.

Daysy VS Fertility awareness method

By learning more about the FAM and exchanging with my advisor, I came to the conclusion that we should distrust fertility monitors.

Indeed, the FAM takes the following factors into consideration:

  • Basal temperature
  • Cervical mucus consistency
  • Cervical mucus appearance
  • Cervix position (optional)
  • External disrupting factors

On the contrary, Daysy only collects and analyzes the basal temperature which is, on its own, not enough to identify ovulation.

This represents a significant risk if the aim is to prevent pregnancy.

Moreover, the body is such a complex organism; it doesn’t ovulate every time on the exact same day and reacts to many external factors such as diet, stress, emotions, etc.

There are so many influences that, in my opinion, can’t be acknowledged by algorithms.

Also, I compared the end of my fertile period according to Daysy and the FAM; Daysy was 4 days ahead.

In fact, when looking at it closer, I noticed that on many green days (= infertile) predicted by Daysy, I actually had a high-quality cervical mucus which is a sign of ovulation.

Therefore, I could have gotten pregnant whilst following Daysy’s analysis. Doing my research, I found that I’m not the only person who experienced this.

I would have liked to continue comparing these two methods but I accidentally dropped the monitor one morning (ok, I was still asleep). It broke and I’m not able to use it anymore, fantastic!

For a 300$ device, it hurts. I was fixed on Daysy and didn’t plan on using it anymore to monitor my fertility, but if I did, it would have been a problem.

fertility tracker daysy

Would I recommend Daysy?

There is no surprise here ; I don’t recommend Daysy as a contraceptive. Here is why:

  • Daysy is only based on basal temperature, which is not enough to define when ovulation happens
  • There is no valid study on its reliability
  • The body is so variable, it can’t be compared to a robot – however, Daysy doesn’t consider real-life factors
  • The monitor is expensive and very delicate

In this article, I mostly talked about the fertility tracker Daysy, but it’s the same for fertility tracking apps.

They can be useful to learn more about your cycle or perhaps to help you conceive… but please don’t take the risk to use it as birth control; they don’t have enough information about your cycle to guide you in a reliable way.

What I recommend instead

If you’re interested in Daysy or that you tried it yourself, it’s probably that you are looking for a natural way to monitor your fertility.

So why not try the fertility awareness method instead?

The FAM is behind all these apps and monitors. Even though all these brands promise yourself to facilitate the process of understanding your cycle, it actually isn’t that complicated!

Yes, you will have to go through a learning period, but once you’ll be done, you won’t have to depend on anyone nor anything to make the right decisions for your body.

No more spendings, no bad surprises if your monitor breaks, no stress if your app disappears one day.




There you have my review on the fertility tracker Daysy, I hope it helped you in some way!

Do you use an app/monitor to track your fertile days? Let me know in the comments below!


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  1. My sister conceived while using Daysy for contraception, but that was because she had baby making sex when her Daysy was flashing red.

    I was spending the night with them at their apartment the night she had her “oops”. On an impulse earlier that day I decided to be nosy and check her Daysy. When I saw it was flashing red, I couldn’t resist the chance to put Daysy to the test and find out just how accurate Daysy really is. I switched her Daysy with mine (which was safely green), and I made sure to empty their box of condoms in the trash.

    I made sure we all had a lot to drink that night, and her husband insisted on having “green night” sex with her when he saw MY Daysy showing green on her nightstand.

    She knew something had to be wrong with her Daysy, and since they were both pretty tipsy, they were loud enough for me to hear her attempt to talk her way out of becoming a mommy that night. The empty condom box ended that discussion! Mother Nature did the rest. Just like Daysy predicted, it was perfect timing for making a baby. I was really impressed with Daysy’s accuracy!

    • Hi Katie, thank you for sharing your (sister’s) experience.
      I agree with the fact that Daysy isn’t entirely wrong and can actually predict fertile days, but it is, according to my experience, not precise enough.
      It isn’t a big deal if the goal is to conceive, but if you’re trying to avoid pregnancy, you could end up with an unwanted pregnancy.

      • You are absolutely right, Virginia. I can’t be 100% sure that the night I did my fun little “experiment” was actually the night she ovulated as Daysy predicted. I do know my sister was being super careful about using condoms. She threw a fit at her husband the next morning, especially after she took her temperature with my Daysy and of course got a yellow light! I am certain that was the only time they had sex without protection anywhere near the middle of her cycle that month. But as you point out, even if she ovulated a couple of days after Daysy’s predicted date, that would still be close enough to do the trick. All I know for sure is that because of Daysy I now have a beautiful niece.

  2. One more comment, Virginia. I still use Daysy myself for contraception, but I do keep an eye on my cervical mucus and my calendar and never rely solely on Daysy. I have made that clear to my partner, too, so he understands why I want to use a condom sometimes even when Daysy is green. That’s the big difference between my sister and me. She knew they needed protection, but she just couldn’t convince her husband to listen to her once he saw that green light on my Daysy sitting on their nightstand.

    • Well, then I am happy if your sister’s pregnancy was a delight for the parents and family and that this story didn’t go wrong!

      The fact that you still keep an eye on your cervical mucus is the best way to go, a double check is always safer! And yes, it also helps a lot when your partner understands and supports you in the choices you make. You sister could maybe try to explain her husband how it all works “behind the scenes” of Daysy, because I can understand that he wants to trust the green lights if that’s the only thing he knows about it. Btw, this is why I didn’t want to use Daysy or any app to track my fertility anymore, because you can easily be biased by colors!

      • That is so sweet, Virginia, thank you! 🥰 The baby is a true delight for the entire family. To be honest, I was a little worried that night when I heard how loudly my sister protested when she realized her husband was not going to take no for an answer and that she was about to have completely unprotected sex at the worst (or for what I had planned, the best 😉) possible time in her cycle. But soon her libido kicked in, and …. ☺️ She’s now pregnant with their third baby without any help from me. All she needed was a gentle little nudge into mommyhood.


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my name’s Virginia

I’m on a mission to help women understand, trust & love their body🦋 

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