candidate: The link between a pregnant mother and the fetus is such that “ at all sites of direct cell - to - cell contact , maternal tissues ( decidua and blood ) are juxtaposed to extraembryonic cells ( trophoblast ) . ” According to Cunningham , " after conception , a biomolecular communication system is established between the fetus and mother that is operative from before the time of nidation and continues through and beyond the time of parturition . "