Sexual reproduction—insemination, conception, implantation, and birth—resembles invasion by pathogens and the consequences are determined, in part, by the host and by the reaction of the host: that is, by the state of health and tolerance of the creature which is “invaded.” There is now growing evidence that normal pregnancy is accompanied by an inflammatory reaction and that typically gestational events, such as preeclampsia, are associated with exaggerations of this maternal “inflammatory reaction.” Furthermore, the maternal immune system, during pregnancy, would react with a sort of immunization to the paternal antigens, capable of protecting against some diseases related to the pregnancy itself. Preconceptional exposure to paternal antigens allows for successful implantation and reduces the risk of adverse pregnancy outcomes associated with increased inflammatory reactions, such as preeclampsia. It is therefore the host, i.e., the mother, who largely determines the concept of high-risk pregnancy.