Natural Cycles bases its prediction for your future ovulation on all of your historical data (such as average ovulation day, and cycle length), LH test results, and your set-up question answers.
Here you can see the icons which will be used to inform you of where the algorithm currently is in terms of calculating your ovulation:
You will be shown an ovulation window with a range of possible ovulation days, such as the graph seen below:
While the algorithm waits for more data from you in order to confirm ovulation, you will see a clock icon in your ovulation window, like this one:
The ovulation window is designed to include your actual ovulation date in almost all cases. However, please note that in a small number of cycles the ovulation date could fall outside of the window.
When your temperature has risen enough to confirm ovulation, the most probable ovulation day will then be confirmed according to the temperature curve.
The algorithm may move the confirmed ovulation day to another day should your newly added data support this. The predicted window of red days is based on the most likely ovulation day and the predicted ovulation window might extend beyond that. If you ovulate towards the later end of your ovulation window, you should then expect more red days. If you ovulate on the earlier part of your ovulation window, you might get green days earlier than previously predicted.
The important takeaway here is that while predictions are a wonderful tool to help show you how the algorithm is forecasting your cycle to be, it adapts and adjusts accordingly to you the more you add your unique data points. Remember to look at your fertility status on a daily basis, as predictions are always subject to change and are not to be used as a final result.
You will see your confirmed ovulation reflected in your graph as a purple ovulation icon like this image below:
Not enough data & anovulatory cycles
It may happen that the algorithm will either not have enough data to confirm your ovulation or conclude that a particular cycle was anovulatory (cycle without ovulation, please read more about anovulatory cycles here). When either of these instances happens, you will see it reflected with an information (i) icon which will appear within the ovulation window as the images below illustrate:
"I did an LH test that showed positive, but Natural Cycles showed that my ovulation is much later than this?"
A positive LH is not enough to confirm that ovulation has happened, nor that it will happen. A positive LH test is a strong indicator that ovulation is close, but a positive LH test must be followed by a temperature increase in order for ovulation to be confirmed. Since progesterone is released when the egg is released and is then "warming" the body, the temperature increase is thus a very important indicator that ovulation actually occurred.
This is a very important function for people with PCOS since one can get so-called PCOS "false positive" LH rises where LH levels rise in the body but the egg is not released (which also means that no temperature increase occurs because the progesterone is not released).
Read more here to find out how Natural Cycles can know whether you've ovulated or not.