How It Works

Lily evaluates your cycles by using the data you provide every day. You can pick between two very different ways to evaluate your cycles, each method requires a different set of data to work.

Read more about the differences between the two:

Evaluation of your cycles based on averages is easy as pie, but less accurate. And even though the evaluation of your cycles based on your body's own symptoms requires a little more effort, Lily will be able to evaluate your fertility with considerably higher accuracy.


When using the averages-based evaluation method, all that Lily really requires is the dates on which your previous cycles started, as well as the length of your luteal phase.
It is important to set the length of your luteal phase correctly, because it is the most important part of how Lily calculates the date of ovulation for your past and future cycles, as well as the fertile and infertile phases for each cycle. If you don't know your luteal phase, you should set it to the default length of 14 days, which statistically fits most women. However, in this case you should not rely on Lily as a method of birth control as your cycle's infertile phases may be calculated incorrectly.


When using Lily's symptoms-based evaluation method, you can pick from two sets of rules that Lily applies to your cycles. Both methods are nearly identical, they both need your temperatures, your cervical fluid quality, when your cycles begin and end, but they differ slightly in their rules and temperature thresholds that need to be met.

If you are not sure which set of rules to use, we would recommend you use 'NFP (NFP AG),' since it has the tendency to evaluate more cycles successfully than 'FAM / NFP'.

Here is an overview over the four rules that are being applied to the data you track with Lily:

You don't have to remember these rules, though. Lily will apply them for you. All you need to do is enter your data daily, and Lily will do the rest. Whenever your data changes, Lily will apply these rules immediately and give you the result right away.

If you should ever wonder how Lily arrived at the result it is showing, you can go to the 'Your Cycles' menu and select the cycle in question. Lily will then show you a detailed list of what rules it applied, along with a lot of additional useful information and a couple of tips and tricks you can use to improve your cycle tracking.

Tapping on the applied evaluation rule will lead you to a second screen where Lily will explain in detail what it did with your cycle, why it used that rule, which temperatures it used to arrive at that conclusion, and what all this means for you. You can even tap on the colored text parts to highlight the corresponding data in the cycle graph above.

There are a couple of things you should know about how to collect the data that Lily needs for its symptom-based cycle evaluation:

When using a symptom-based evaluation method, the absolute minimum that Lily needs to evaluate your cycles is your temperatures. Without tracking them regularly, Lily will not be able to tell you when you should be fertile or infertile, and will not be able to predict your future cycles accurately.

Even though Lily doesn't require it per se, you should still consider tracking your cervical fluid as well, especially if you are trying to avoid a pregnancy, as your cervical fluid is being used to counteract any ambiguities that may arise from the temperature evaluation results.

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We do not guarantee the method of birth control or pregnancy achievement as described on this web site.

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