About what percentage of natural conceptions fail to implant properly?

About 60% of natural conceptions fail to implant properly. One of the reasons may be the failure of the embryo to generate Trypsin, a chemical which induces a receptive environment in the uterus in order for the fetus to be implanted. The excess or lack of chromosomes of the fertilized egg may also be a factor in implantation failure. A weak immune system of the woman may also be an important factor.