Pregnancy rates do not prove that the pill does not prevent implantation. Pregnancy rates could be affected by missed pills, by interacting medications, by poor absorption, or by the chance event that a woman had break-through ovulation and conceived and then the baby implanted in the uterus successfully. What we do know is that the pill affects implantation in many cases and usually thins the endometrium. So we know that in some cases implantation is impeded.