Sorry, Jill, you are simply wrong here.
Did you know the birth control pill can cause chemical abortions?
No, probably not, unless you're a radical pro-lifer like me, or a radical pro-abort. We all know.
I've been interested in this topic for a number of years, and have reviewed much of the medical literature on the topic. I often joke that I am probably the only oral and maxillofacial surgeon who subscribes to the journal Contraception, which I do in order to have electronic access to all of their back issues. In short, I have done my homework here, and can say without a doubt that Jill has seriously overstated the evidence we have for a post-fertilization effect from OCs. I would summarize the evidence we have about the mechanism of OCs in this way:
1. We do not have full knowledge of the actual mechanism of OCs, and there is little prospect that we will gain this knowledge in the near future. There are questions that we simply do not know, and will almost certainly not know anytime soon.
2. There is no direct evidence that OCs cause "chemical abortions". In other words, there is no direct evidence that OCs effect the uterine lining to cause an embryo that would otherwise implant not to.
3. There is indirect evidence that causes some to believe that OCs can cause changes to the uterine lining that would decrease the chance of embryo implantation in the cases in which breakthough ovulation occurs. This indirect evidence includes differing ectopic pregnancy rates with those on OCs and the thinning of the utering lining for women on OCs. I am well aware that some pro-lifers are persuaded by this indirect evidence that OCs cause a hostile endometrium in a minority of cases (like Randy Alcorn and Dr. Walter Larimore). However, there are also a number of pro-lifers that are unconvinced by this indirect evidence - including myself. If prompted by comments, I will go deeply into the literature on this topic to explain my reasoning. In the meanwhile, I would recommend the book "The Pill" by the Center for Bioethics and Human Dignity for good summaries of both sides of this argument.
4. Direct evidence does not exist that would prove that OCs can never act via a post-fertilization mechanism.
5. Both the package insert and the PDR lists the hostile endometrium theory as a possible mechanism of action of the pill. This is not surprising, considering #4 above. For more information on the accuracy of package information and the FDA, see this post here.
There is a real controversy regarding this issue, and science does not have all of the answers. Let me state this plainly: anyone who believes they know absolutely that OCs cause endometrial changes that result in "chemical abortions" is simply wrong. They don't. I don't know for sure either. Cool web sites and T-shirts do nothing to change this fact. Pro-lifers who overstate this case are acting very irresponsibly.