I recommend that you read this paper. Modulation of the maternal immune system by the pre-implantation embryo. Caroline G Walker; Susanne Meier; Mathew D Littlejohn; Klaus Lehnert; John R Roche; and Murray D Mitchell DOI: 10.1186/1471-2164-11-474 Mendeley URL: http://www.mendeley.com/research/modulation-of-the-maternal-immune-system-by-the-preimplantation-embryo/