"Why would abortion rights advocates play such a verbal trick? The redefinition of this one key word has given them license to lie to millions of women by telling them that the birth control pill, Norplant, Depo-Provera, and other such chemicals do not cause abortions.
The truth -- according to medical science as well as the pharmaceutical companies that make the products -- is that chemical birth control methods can, and do, cause abortions. How does this deception work? Since according to the pro-abortion community, ``conception'' now means ``implantation,'' contraception (against conception) is anything that works before the newly fertilized egg implants in the uterus. By this definition, the birth control pill is acting as a contraceptive when it (according to a pill manufacturer) ``makes the womb (uterus) less receptive to any fertilized egg that reaches it.''
The birth control pill can act as an abortifacient -- it can kill human life -- but is falsely labeled a contraceptive."

Some more science.