The Chicago Tribune has an interesting article on emergency contraception, also known as Plan B or the morning after pill.
The article seems to chastise people who believe that emergency contraception may act to prevent the implantation of an embryo because there is no scientific evidence to back this up while at the same time the main proponent in the article for emergency contraception, Dr. David Archer, states that he can't prove that emergency contraception may prevent implantation. It seems no one is certain of exactly how this pill works or if it works when ovulation has already occurred.
Notably, there hasn't been a lot of research on Plan B. The article lists a review of literature at the Karolinska Institute in Stockholm and a study from Horacio Croxatto of the Chilean Institute for Reproductive Medicine that found emergency contraception "interfered with ovulation 82 percent of the time in women who took it."
That begs the question - what happens the 18% of the time that emergency contraception doesn't interfere with ovulation? Or what happens when ovulation has occurred before the woman takes the pill? Does the pill not work at all then?
The article also states, "But in their review of the studies on emergency contraception, the Karolinska researchers found that one-time treatment with levonorgestrel had no effect on the uterine lining." I'm guessing this means at the time when the pill is taken and not long term effects but the article doesn't elaborate. It seems that if researchers could prove that the pill doesn't effect the lining of a woman's uterus when the pill is taken, they could prove that the pill doesn't prevent a human embryo from implanting.