Its been a while since I posted on Plan B, so I'll summarize my thoughts so far:
1) an analysis of the most recent literature supports the theory that Plan B works predominately, if not exclusively, prior to the fertilization event. In other words its effectiveness is primarily associated with its ability to impair ovulation, and not effect implantation.
2) If Plan B does not work after fertilization, the reported effectiveness of the medication must be less than the 85% or 91% that has been reported in the literature and on the package insert.
These two posts address the effectiveness issue of Plan B. If I am right here, we would expect the data that uses a more accurate measure for predicted pregnancy rates to show decreased effectiveness for EC. I have detailed four studies which showed that real world usage of Plan B does not effect pregnancy rates. However, one could argue that factors outside of medication effectiveness were responsible for the number of pregnancies in the Plan B groups. When I wrote the original posts, there was no studies out which showed a decrease in the effectiveness of Plan B using a more accurate way of estimating pregnancy risk. As I will detail in the next post, that is no longer the case.