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 a woman’s 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.