If.
As the article explained, this is not the (calendar-based) rhythm system of the 1930's. That was your great-grandma's method.
Anyway, modern fertility-based methods are about real-time observation of each woman's particular, individual fertility signs so she can either avoid or achieve pregnancy.
It's not about calendar average cycles at all, because few women have calendar-average cycles.
Right, great-grandma? (Chuckle.)
The rhythm method worked great for me too.
Loved it.
I tried to read the article but kept having pop ups so I closed it.
I hate websites like that.