It appears that the new contraceptive "morning-after" pill called Plan B cannot cause an abortion, due to the fact that when it prevents a pregnancy it does so by suppressing ovulation. (Thus, it's not effective if you have sex after ovulation has already occurred. This method of operation makes Plan B morally superior to RU-486, which can act as an abortive.
Several articles published in the scientific journals Contraception and Human Reproduction have found in rats, monkeys and humans that Plan B, or levonorgestrel (the active ingredient in levonorgestrel is progestin), prevents ovulation without interfering with fertilized eggs.
In contrast, mifepristone, known as Mifeprex or RU-486, is a drug that will induce abortion in the first 49 days of gestation. FDA regulations require that women take the drug only under a doctor’s supervision. Another drug, methotrexate, was originally developed to treat cancer and is also sometimes used to induce abortion.
I've written about RU-486 morning-after pills before and argued that if a woman uses the drug to prevent an implantation it shouldn't be considered as equivalent to abortion; ending a pregnancy after implantation is the real problem. Zygotes fail to implant naturally all the time, and regular birth control pills (also progestin, the same drug as Plan B) can occasionally fail to stop conception but then successfully prevent implantation (possibly, though the evidence is slim). So it goes. Even if human life "begins at conception", it is part of the natural process for zygotes to frequently fail to implant in the uterus, often because they are malformed or unhealthy in some way.