While viviparity has evolved many times in vertebrates and even several insects and velvet worms, the fetal maternal relationship in eutherian mammals (placental mammals) stands out as unique. To my knowledge, there is no other animal group where the fetus is affecting the maternal tissue to such an extent as in eutherian mammals. The situation, however, is complicated by the fact that eutherians present a variety of fetal-maternal relationships, from highly invasive hemochorial placentation to largely “non-invasive” epitherliochorial placenation in what colloquially is called “farm animals” and their relatives. A key insight from modern molecular phylogenies is that the most recent common ancestor of placental mammals had an invasive placentation and that the less invasive placentation present in farm animals is a secondary development. The key problem then is to understand how the evolution of invasive placentation and extended gestation in the stem lineage of eutherian mammals was achieved at a mechanistic level. Based on our research on the gray short tailed opossum, Monodelphis domestica, I will present evidence supporting the following scenario: the therian ancestor; that is, the most recent common ancestor of marsupials and eutherians, had a situation similar to that as it exists in the opossum, where the embryo remains separated from the uterine tissue by a shell coat for most of gestation, and once hatched, it remains attached to the uterus only for a very short time. A consequence of fetal attachment is an acute inflammatory reaction on the part of the mother, which directly leads to parturition. The key innovation, necessary to evolve the extended gestation of eutherians, in spite of the invasive behavior of the conceptus, is a mechanism for controlling the damaging consequences of inflammation, in particular neutrophil invasion. This was achieved by 2 innovations, the evolutionary origin of the decidual cell, which helps control the inflammatory process and the dependency of the decidual cell on progesterone. I will discuss the implications of this model for understanding and managing pregnancy complications in eutherian mammals.