Approximately 50 percent of pregnancies in the United States are unintended, according to a February 2014 report in the American Journal of Public Health. Plan B is one of several methods of emergency contraception to prevent pregnancy after unprotected sex or a birth control failure. It contains the progesterone-like hormone levonorgestrel, taken in 1 or 2 doses. Plan B reportedly prevents roughly 7 out of 8 unintended pregnancies when taken within 72 hours and is even more effective if taken within 12 hours.
Plan B reduces the risk of pregnancy by roughly 89 percent if taken within 72 hours of unprotected sex, according to an April 2014 review article in BMC Women's Health. Plan B is even better at preventing pregnancy if taken within 12 hours after intercourse, according to a February 1999 report in The Lancet. While this study showed a clear trend for better pregnancy prevention with earlier Plan B treatment, there are no precise, reliable estimates of how effective the treatment is within 12 hours.
Delayed Treatment Still Helpful
The earlier Plan B is taken after unprotected sex, the better it is at preventing pregnancy, but it maintains some effectiveness for up to 120 hours, or 5 days. According to the 1999 study in The Lancet, pregnancy rate continues to rise the longer emergency contraceptive treatment is put off, nearly doubling every 12 hours treatment is delayed. Thus, while Plan B treatment is still potentially beneficial for preventing pregnancy after 72 hours, it is not as effective.