Physiology of implantation

Implantation is arguably the most critical stage in the establishment of pregnancy. In humans, it has been estimated that between 30% and 70% of conceptuses are lost before or at the time of implantation, without women being aware that they were pregnant. Of these losses, half are probably a consequence of genetic defects in the conceptus. The etiology of the remaining losses is unknown. In human in vitro fertilization and embryo transfer programs, the low implantation rate after embryo transfer is an important problem. Whether this low rate is a consequence of an inherently low implantation rate in humans, or to an altered physiological state is presently unknown. Based on studies in experimental animals, there are reasons for suspecting that the low implantation rates may be a consequence of an altered physiological state.