The processes of deciduation of endometrium prepare the appearences of a structure which reprezented by decidua. This khew structure will regulated acceptation graft. Reprezented by pregnancy through specifi c immunologic mechanisms, which add endocrine and paracrine functions of decidua. The resence in decidua structures of numerous cytokine and cells with particular structures and activity, will permite trophoblastic invasion and changes to the interface level of mathernoplacentar. Profound study on decidua will lead to undestand with very high fi neness the mechanisms which is stoy on the basis of trophoblastic invasion.