When you create an account with Natural Cycles, you will be asked to register an email address and select a password.
You can see which email address you have registered on your account by going to the top menu > My account > Email. There, you can also confirm your email address and change it if needed.
If you are unable to log into your account, your registered email address is also on the invoice that was sent to you after your purchase.
I do not see my email address in the Natural Cycles app
If you select Sign up with Apple when you register your NC° account, a unique, random email address is created, which can only be used for Natural Cycles and can't be reused for other apps or services. You can read more about this in this article What is Hide My Email?
This email address is unique to you and follows this format:
<unique-alphanumeric-string>@privaterelay.appleid.com
While you will still receive all our emails via your personal email address, they will stay private and not be shared with the app or website developer. This explains why your private email address will not be visible in the Natural Cycles app.
If you need to reference this Apple email address, you can find it by going to the Settings app on your iPhone or iPad > tap your name > Password & Security > Apps Using Your Apple ID > Natural Cycles.
Note: If you are struggling with logging into your account, please check out this article Why is the app asking for payment when I have a subscription?