I am a family physician and professor at Oregon Health & Science University in Portland, Oregon. I am also one of the physicians on the Physicians Resource Council (PRC) at Focus on the Family. I do not prescribe oral contraceptive pills (OCPs) or birth control pills (BCPs) for the purpose of preventing pregnancy.
At the PRC we discussed the potential abortifacient properties of OCPs/BCPs for more than three years. We examined all of the literature available and the bottom line is that we could not develop a clear consensus. One or two physicians felt that these pills never cause an abortion in any woman at any time, some felt that they did rarely (if ever), some felt that they clearly could and would in at least some women in some cycles. For those of us in this latter group, the frequency with which this occurs is clearly going to vary depending on the amount, potency and mix of hormones.
For more information on the subject of oral contraceptive pills, see William Toffler's article "Why does Focus on the Family take a stand against Emergency Contraception (EC) but not theBirth Control Pill (BCP)?" and Randy Alcorn's book Does the Birth Control Pill Cause Abortions?