I have been gradually teaching my girls this, as they get old enough to understand. I think it is a huge travesty that girls are not taught this at a young (fertile) age.
It’s very easy, if you start by simply having them record their dates of menstruation, as soon as it’s relevant. If they have unusual results, then observing temperature can often be informative.
The menstrual cycle is a general reflection of a woman’s health from puberty to menopause, and it’s very unfortunate that so few women, and so few doctors, pay any attention to what it reveals.