The "morning after" pill, which is a type of emergency contraception, works by preventing the release of an egg from the woman's ovary; if no egg is released, pregnancy cannot occur, according to Planned Parenthood. Contrary to popular belief, pregnancy does not occur right after sex, but instead it takes up to 6 days for the egg and sperm to meet after sexual intercourse. This type of emergency contraception is different from the so-called "abortion" pill, and is birth control only; it does not cause an abortion, since there's no pregnancy to abort.
When taken with 72 hours of sexual intercourse, levonogestrel pills, which are "morning after" pills sold under the brand names Next Choice One Dose and Plan B One-Step, are up to 89 percent effective. They also continue to reduce pregnancy risk for up to 120 hours following unprotected sex. Their effectiveness reduces as time goes by.
It is important to note that this type of emergency contraception will not prevent pregnancy that occurs due to sex after taking the "morning after" pill. In addition, levonogestrel pills may be less effective for women with body mass indexes, or BMIs, that are greater than 25. For these women, other forms of emergency contraception may be better options.