Gud eve mga mommies! Cno po ang gumagamit ng calendar method? Effective po ba ito? At paano?

ngtry ako kami ng calendar method using a period tracker app, so na count ko po yong length ng period at fertile days. my lmp was aug. 1 i went to manila aug. 14 - 20(fertile days according to the app) and have an unprotected sex with hubby. and now i'm 5 mos preggy n po. if trying to avoid pregnancy, ma te- trace mo talaga when is your safe dates