How the Natural Cycles algorithm works

Today’s blog is about how the Natural Cycles’ algorithm works, aiming to answer questions like why upcoming fertility predictions can vary and why ovulation day can differ from one cycle to another.  We also wanted to give our Cycler’s an exclusive sneak peek at the new app interface that will be released later this year. Please let us know your thoughts!


The Basics

The Natural Cycles’ algorithm works by calculating your fertility based on the temperature readings and LH tests you enter into the app from the very first measurement up to the most recent.


Both the weekly and monthly forecast’s found on the calendar view are fertility predictions (indicated by question marks and hollow circles) that are completely tailored to each individual, mapping your unique cycle and important events, like ovulation, with the highest precision possible from the data you have already entered into the app.


However, like all predictions, your upcoming fertility can change as unfortunately we cannot see into the future!  With every new data entry, the app’s algorithm adapts and adjusts accordingly to help paint the clearest picture of what you can expect in the days to come.  This is why we always say check your fertility status each day – especially around ovulation – as it can differ from what was forecasted the week before!



In this calendar, ovulation day is predicted on Wednesday but according to the data you enter the exact day can vary. Once ovulation is detected it will be indicated with a blue egg icon on a complete red circle.


Data entry and cycle variations


A key determining factor for forecast variations is data entry.  The more data a Cycler enters the better the algorithm can be at determining your cycle’s usual pattern and thus predict what is coming up. So gentle words of encouragement – try and measure as much as possible to improve your data!


Cycle variations, such as a delayed ovulation day can be another reason for changes in your calendar. Cycler safety or their success is our priority so if the app detects something unusual it adapts by either a) giving more red days to be sure you’re protected from the risk of pregnancy (prevent) or b) advises you to keep trying as you’re likely to be highly fertile (plan), respectively.


Below is the in-app notification for Prevent users that will pop up if your ovulation is delayed.


Screen Shot 2015-10-08 at 16.30.09


Alternatively, for Plan users this  in-app notification will be displayed if your ovulation day is delayed. 


Screen Shot 2015-10-09 at 15.37.27


Cycle variations are common as the follicular phase (before ovulation) often varies in length. For instance, if you ovulated on day 18 on your previous cycle, it doesn’t mean that you will this cycle or even next.


A Cycler will only get green days once it is clear you have ovulated (with a clear rise in temperature) and is confirmed by the app. The app confirms this with a blue egg icon on a filled red circle on the calendar view or a blue egg icon and blue line seen on the chart/graph (see below).


Ovulation day was confirmed on cycle day 16 indicated by a blue egg icon and blue line on the chart/graph view.

Chart/graph view: Ovulation day was confirmed on cycle day 16 (CD16) indicated by a blue egg icon and blue line.


Ovulation day has changed


A couple of observant Cyclers have noticed that when looking back at their previous cycle their ovulation day has changed by a day or so. This is because the algorithm is constantly trying to calculate the exact day of ovulation even after it has already happened! Once your luteal phase has ended, which is indicated by the start of your period, the app has more information to work from.  With this extra bit of information, the app really does know when you ovulated and uses this to create an even more accurate picture of when you will ovulate this next cycle.


You can see that the app is continuously working very hard to get to know your cycles – variations and all! Natural Cycles adaptability to women’s cycles no matter what shape or size they are – long, short, irregular – is what makes it suitable for almost anyone. You can be assured you are always protected.


How will the new app differ?


The new app, that will be released later this year, will be even more intuitive and easier to follow. You can see below that it has been designed so that your fertility for the day is the core focus.


The upcoming fertility predictions will only be visible if you request to see them via the ‘Tap to show’ button. This way we can ensure that you feel confident about your actual fertility status instead of relying on predictions that may change.










Let us know what you think?

The Natural Cycles Team



How do I use my NaturalCycles Statistics?

When using NaturalCycles for the first time you might ask yourself “How do I use my NaturalCycles Statistics?”, especially when you’re new to terms such as luteal phase and your statistics are looking a bit incomplete.


No numbers yet to be shown!


But after measuring for sometime (usually after 1-2 cycles) and after NaturalCycles has detected your ovulation day, you can expect your statistics to look a little more like this:

Screenshot 2015-04-23 15.29.30

Below you can read what all the signs/numbers mean. (Click on the picture to make it bigger)

Let us know if any questions come to mind – email us at



Your Cycles Statistics!

Everything you need to know about the Natural Cycles basal thermometer

To get started with Natural Cycles you will need a basal thermometer. It’s more sensitive than a regular fever thermometer as it shows two decimals.

Here are the most useful tips on how and when to measure your temperature with it:

basal thermometer, naturalcycles, natural cycles

The basal thermometer!

Why Basal Thermometer?

A basal thermometer is a digital thermometer showing two decimals, (for example 36.99). It is much more sensitive than a regular thermometer, as it measures the slightest changes in your temperature. Something that is very important when measuring your basal body temperature, which rises in very small amounts near ovulation.

When to measure?

Measure first thing in the morning, even before you get out of bed! Why? The basal body temperature is our resting temperature and is lowest when we sleep. The slightest movement (getting out of bed) will increase your body temperature, thus making your reading further away from your actual basal temperature and therefore slightly less accurate. In fact, we advise NaturalCycle users to try and measure at roughly the same time every day, within a ±2 hr window of your usual morning alarm. If you decide to have a lie-in after a busy week at work then we suggest you leave it for the day.

How to measure?

howtomeasure, naturalcycles, thermometer, basal 

I’ve measured twice and thermometer shows two different temperatures?

Our basal thermometer’s are super sensitive in order to capture the minute changes in your temperature. With every reading your body’s temperature will naturally rise, and so the first reading is likely to come out lower than the next. There is no need to measure twice as the first reading will be the closest to your actual resting temperature.

What is LO?


‘Lo’ means low temperature, and not low battery – so no worries! The reason it says ‘Lo’ from time-to-time is because room temperature is roughly 21 degrees, which is ‘Lo’ for our basal thermometer. Body temperature should be around 36.5–37.5 degrees (a little science for you). Any higher than this, then you probably have a fever and therefore advise you to leave the reading for the day.

A big pointer for all you newbies! It will take a 1-3 cycles of measuring for Natural Cycles to get to know your unique cycle.



Your Natural Cycles team


Optimized algorithm = more green days!

Exciting news! NaturalCycles has further optimized the algorithm with all the awesome new data streaming in on a daily basis.

Thanks to more than 40 000 days of collected data we are now giving 10% more green while remaining 99.9% safe. You can now see the new results in both your fertility history and the predicted red and green days.

Check it out in your app (AppStore / GooglePlay) or on your MyCycles page.


Optimized algorithm

Newly designed NaturalCycles app for iOS in App Store!

The newly designed NaturalCycles app for iPhone and iPad is now approved and in the App Store. Update your apps or download it for free here!

You get a three month free Plan/Prevent trial.

Newly designed NaturalCycles app for iOS

NaturalCycles iOS App submitted

Hi everyone!

Today we finally submitted our NaturalCycles iOS app for approval for being released for iPhones and iPads in the Apple Store.

The procedure to submit an app to Apple is quite extensive, with many configurations to set and certificates to create. That’s why we are so happy today that we’ve completed all of this work, which not to forget also includes developing the app itself. Now comes the real review process on the Apple side, which can take up to 6 weeks.

Our app has the same structure as the NaturalCycles website for the logged in user – it is different for NC Free, Prevention and Planning. The app itself is free, but one needs to register for a NaturalCycles account to use it, either on the app itself or on Similar to our website, the whole app changes its color scheme depending on whether you are having a red (fertile) or green (not fertile) day or if you are pregnant, which is when everything becomes blue.

iOS App - NaturalCycles

iOS App - NaturalCycles

iOS App - NaturalCycles

iOS App - NaturalCycles

In addition to the fertility calendar and temperature chart, the app also has the analytics summary of your cycle and a pregnancy tracker if you are pregnant. It also contains Prevention and Planning discussion forums, where you can chat with other users and share your charts, etc.

I think the app is a very nice addition to our website, since NaturalCycles is something you use on a daily basis and not everyone sits in front of a computer all the time, like I do. The app is made with a cross-browser framework called PhoneGap, such that it works for all device dimensions and should be easy to duplicate also to other platforms. Hopefully the Android app will not come long after the iOS app.