text: surely its more to do with the effectiveness of the pill. in britain you can take it upto 72 hours later, but if you wait that long its only 50% as likely to prevent pregnancy than if you take it straight away.\n\ni think its more to do with the fact that the longer you leave it, the more likely it is that the embryo will have planted itself in the uterine lining, and that the body will have signalled the release of progesterone - the pregnancy hormone. after this time, you are pregnant, and only a much stronger pill that will cuase contractions of the unterus to dispell the embryo, will work.