If your cycles are regular, then after Flo makes an ovulation prediction, it builds the fertile window the following way: 4–5 days before ovulation, the ovulation day, and 1–2 days after ovulation. The minimum fertile window will be 7 days. These calculations are based on the fact that sperm can survive in the female body for up to 5 days and the egg is viable for up to 24 hours.
If you have irregular cycles or haven’t logged at least three cycles, Flo doesn’t have enough information to make a precise ovulation prediction. This means the application will build a longer fertile window to try to avoid making mistakes. However, when Flo has enough information about you, or your cycles become more regular, the fertile window will become shorter.
You can get a quick overview of your previous cycle durations in Menu > Graphs & Reports > Cycle Length/Cycle Duration.