This brings up several good issues.
The first is how to use science in political and religious level discussions. The issue is how to draw the lines in the ethical and moral realm, simply defining abortion as anything that stops or disturbs implantation as well as a medical procedure that takes measures to end a pregnancy is to confuse the topic and to align very different things as if this logic was scientific and not part of a moral discussion. IUD's can stop/hinder/disturb implantation, but this is not the same thing as abortion. if you wish to argue that all methods that hinder a fertilized egg from implanting and developing then say that. Not hiding behind such a loaded term as abortion to prove your point by definition. If you are against any controls that hinder fertilization, than argue with those words as well, and stop hiding behind definitions.
The second is how to understand the science at it's own level and on it's own terms. The fact is that there is a continuing scientific controversy over the exact mechanism of IUD's. There maybe several, for each kind of IUD. To tell people that their science is wrong and therefore their moral decisions, is not doing justice to the controversy that exists on the scientific level. That IUD's can stop the sperm from getting to the egg is not a discarded and wrong scientific theory but rather one under activity study.
something a few minutes spent googling would have informed anyone
from:
http://www.ppscny.org/birth_control_quick_facts.htm
i read a number of BC sites and they all referred both to hindering implantation and fertilization as a means of action for IUD's.