Natural Famlily Planning (you date yourself by calling it "rhythm," a term which hasn't been used since the 60's) isn't contraception. It's abstinence.
Not for three weeks, it ain't.
Oh, and when practing NFP to decide when to "abstain," watch that whole digital manipulation of the cervix. Some NFP practitioners warn that this amounts to masturbation.
I am not, repeat, NOT, making that up.