Emergency contraception helps prevent pregnancy before it starts after unprotected sex or birth control failure. Must be taken within 72 hours (3 days) after unprotected sex or birth control failure. The sooner it's taken, the better it works. Works by temporarily delaying ovulation or the release of an egg from the ovary. No egg. No fertilization. No pregnancy. Will not work if you are already pregnant and won't impact an existing pregnancy. And, it doesn't affect your ability to get pregnant in the future