In order to use an Apple Watch with Natural Cycles, your watch must have a temperature sensor. This includes Apple Watch Series 8 and later, or any model of Apple Watch Ultra.
Please note the Apple Watch SE does not have a temperature sensor, so it can’t be used with Natural Cycles.
If you have an Apple Watch but are not sure which model you have, this article may be helpful.
Learn more about how the Apple Watch works with Natural Cycles here.