There is no question that all hormonal forms of contraception have the potential to thin the lining of the uterus. If this thinning is significant, a fertilized egg that reaches the uterus could fail to implant, and be destroyed. However, if a method completely prevents ovulation, there is no fertilized egg to be destroyed, and the thinning of the uterine lining is a non-issue.
In the past we felt that the data was insufficient to prove that any method completely stopped ovulation. We now feel that such evidence does exist for some methods, and that there is no pro-life reason to avoid these methods. Additionally there is clear evidence that some methods do not completely stop ovulation, and there are methods for which we have not yet found compelling evidence either way.