Breast-feeding as an abortifacient?

I came across this citation while reading a William  Saletan column, which, if I can decipher the jargon, indicates that ovulation may still occur during the postpartum breast-feeding stage. Does this suggest that, during this stage, a woman may have a fertilized egg that does not implant due to breast-feeding? This would place breast-feeding as an abortifacient practice in line with other methods of contraceptive unacceptable to pro-lifers.

Saletan's earlier column does a nice job of capturing the scientific uncertainty over what happens with eggs and implantation with emergency contraception (like Plan B).