Peters concludes that the assumption that Plan B impedes a human embryo from attaching him/herself onto his/her mother's uterine wall is "incorrect" based on current research.
The first consists of studies that have observed the changes in human female chemistry and reproductive anatomy when women take levonorgestrel. A thorough review of these studies by researchers from the Karolinska Institute in Sweden concluded that Plan B, taken as an emergency contraceptive, does not cause changes in the uterine lining but delays or blocks ovulation, probably by impeding the surge of a hormone that triggers it.
In other words, it does not interfere with the implantation of a fertilized egg.
— The second area of research measured implantation rates directly, which required experiments on animals, rather than humans. Researchers at the Catholic University of Chile specifically designed two studies, one on rats and a later one on monkeys, to detect whether Plan B disrupted implantation. Neither study found any evidence of any such disruption.
Indeed, when Plan B was administered after fertilization, pregnancy rates were the same for the females given Plan B as for those given a placebo.
Though he concedes the studies aren't conclusive he points out that we should base our opinions regarding Plan B on scientific studies and not mere assumption.