EC is usually quoted as being about 89% effective, but it is more effective the sooner you take it, with some effectiveness for 5 days (120 hours). It generally prevents ovulation, so the sooner you take it, the better chances you have of preventing that. (Unless you've already ovulated, which gets into murky territory.) If you are already pregnant - with a fertilized egg implanted in the uterus - EC will have no effect.