Hi - the app gives you probabilities depending on the information you've provided. However, every woman's body can go through unexpected irregularities due to any number of things, including things like illness, stress, or medication. So, it analyzes the information you've provided and tells you when you're most likely to be fertile and when your period is mostly likely to start, but it's not absolutely predictive of your precise hormonal situation, if that makes sense. It's a great tool to help you analyze when ovulation is most likely to take place and when it might be feasible to take a pregnancy test, as well as to keep track of your general menstrual cycle and symptoms that are associated with its various phases. Good luck on your journey!