First Ava showed me a fertile window of 6 days, and then it changed to 4 days. Why?
When Ava detects the very first signs that your fertile window is opening, it automatically projects a fertile window of 6 days, since this is the typical length between the first signs of fertility and ovulation. But if ovulation is detected earlier than predicted, the algorithm will adjust. This is why you may occasionally see a 6-day fertile window at first that subsequently changes to a shorter window.