Plan B One-Step emergency contraception helps prevent pregnancy before it starts after unprotected sex or birth control failure. Plan B must be taken within 72 hours (3 days) after unprotected sex or birth control failure. The sooner it's taken, the better it works. Plan B works by temporarily delaying ovulation or the release of an egg from the ovary. No egg. No fertilization. No pregnancy. Plan B One-Step 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.