If you don’t get any periods, we don’t recommend using Natural Cycles. The algorithm wouldn’t be able to analyze your cycle, and you would only receive Red Days. Your period marks the start of a new cycle and is one of the key indicators the algorithm uses to calculate your fertility status.
Even if you still ovulate (for instance, if you had a hysterectomy but still have your ovaries), the algorithm would not be able to identify your fertility status without period data to differentiate between your cycles.
You can learn more in this article How Natural Cycles works, and you will find more information on Natural Cycles and the menstrual cycle in the article What is the menstrual cycle?
Note: If you have irregular periods, check out our article Can I use Natural Cycles if I have irregular cycles?