Most of the women I know well enough to know this about them track and predict the onset of their next period without needing an application. It isn't exactly rocket science.
The trick is to "give a tool for 1-2 generations of customers" , and then they'll be fully dependent on the tool.
kids today cannot navigate without turn-by-turn. nobody looks at the map to get names of major streets, they just blindly follow the directions. I learned how to navigate as a kid just by being bored and staring out the window and being able to recognize things. Now, kids don't even look out the window as they keep their heads down and eyes glued to a screen.