[Aquinasfan]

No one has done the experiments to show that oral combination contraceptives cause abortions

It's my understanding that "the pill" thins the uterine lining which in some cases serves to prevent implantation, thus causing an abortion.

[hocndoc]

The theory that the uterus lining is thin isn't proven in the case of those months when women ovulate. Not to mention that something must have gone wrong in the hormonal milieu to allow ovulation, but the corpus luteum makes higher levels of hormones than those present in the pill.