Is it effective to use condom in sex to prevent pregnancy? and do sex on infertility dates following the calendar method?

Usually/maybe: Used appropriately (proper size,application before any intercourse) a condom is relatively good at preventing pregnancy. Using a calendar to predict fertility is a good way to improve chances of getting pregnant, but not so good to avoid it.During training we called it Vatican Roulette, because its just another game of chance, and you can lose big.