When you first download the app, it will ask you a series of questions including when your last period was.
Knowing when your last period was helps establish some background about your cycle history and helps the algorithm know which cycle day you are currently on, in order to display the information appropriately.
By entering a period, the algorithm is able to automatically project your upcoming fertile window and your predicted ovulation date, though this can change in real-time depending on what the algorithm is detecting from your physiological signs.
If you don’t remember when your last period was, you can simply select “I don’t know”, and when your next period comes, you can enter those dates into the app by selecting the + icon.