The peri-implantation period extends from the time the blastocyst is free in the uterus, through the processes of recognition and attachment, to the beginning of trophoblast differentiation and the interactions between the embryo and the uterine endometrium which initiate establishment of the hemochorial placenta. It is during the peri-implantation period that the embryo and hormonally regulated endometrial cells appear to be most sensitive to factors which introduce risk into the intrauterine environment. There are no markers which can be used practically to assess pregnancy risk during the peri-implantation period of either human or laboratory rodents. Experimental studies, using in vitro laboratory models of differentiating trophoblast cells, have identified peptide hormone markers of pivotal developmental processes. Exposure of trophoblast during the expression of these processes could have severe and far-reaching effects individually and societally. While these trophoblast signals are limited in their utility with respect to health monitoring extrapolation of these findings to human pregnancy, the signals could serve to identify more practical and sensitive markers to assess risk in early gestation. Human chorionic gonadotropin (hCG) has been used extensively as a marker to assess risk during the early stages of pregnancy. Extrapolation of experimental data indicates how hCG could be used more effectively in analyses of possible cause and effect relationships. The limitations of hCG as a marker for risk during the human peri-implantation period are discussed. Peptide hormones which could serve to assess risk during this critical period of extraordinary sensitivity to toxic factors are introduced.