Before your ovulation is confirmed
The algorithm will make a prediction for your upcoming ovulation day in each cycle based on your historical data, such as when your ovulation usually occurs and how much this varies from cycle to cycle. You can view your own average ovulation day and variation in your Cycle Insights (accessible from the Today view > See cycle insights).
The predicted ovulation window represents the days when the algorithm believes your ovulation may occur in this cycle.
The predicted ovulation day will be shown in your graph with a gray ovulation symbol, and the predicted ovulation window will be displayed as a gray field on the surrounding days.
In the graph above, we can see a predicted ovulation window between CD 14-20, with a predicted ovulation day on Cycle Day 17 (CD17).
After your ovulation is confirmed
When the algorithm has confirmed that your ovulation has occurred in the ongoing cycle, it will calculate the most likely ovulation day from the temperature curve. If the algorithm is not able to narrow down one specific ovulation day based on the data added, it will provide a window of several days within which your ovulation most likely happened.
This ovulation window will be displayed as a purple field in your graph, and the most likely ovulation day will be marked with the ovulation symbol.
In the graph above, we can see an ovulation window between CD 15-17, with the most likely ovulation day on Cycle Day 17 (CD17).
As you continue to add more data during your cycle, the algorithm may also come to recalculate the day you most likely ovulated if your newly added data supports this.
If the algorithm doesn’t have enough data to narrow down the exact ovulation day before the end of the cycle, it may be able to do so once you start a new cycle. Since your luteal phase usually remains rather stable from cycle to cycle (although the length can vary from person to person), your average luteal phase length gives some additional information about the timing of your ovulation.
When a new cycle starts, the algorithm will go over all the data for the past cycle one more time and also consider your luteal phase. It will then be able to narrow down the ovulation window to one specific day, considering both the temperature curve and your average luteal phase length.
Continue to read about ovulation detection here.