The evolution of live birth in vertebrates requires the formation of a placenta, which is an organ formed by the apposition of maternal and embryonic tissues to support the exchange of materials between mother and embryos during pregnancy. Whilst the first mammals were egg laying, live birth evolved in the common ancestor of marsupial and eutherian mammals approximately 170 mya. In this common ancestor pregnancy is believed to have been short, with mothers giving birth to small, under-developed young. This mode of reproduction has been retained in marsupial mammals. To understand the genetic processes that support the formation of the placenta in marsupials we measured transcriptome wide gene expression in uterine tissues of non-, mid-, and late-pregnant grey short-tailed opossums (Monodelphis domestica). On this data we performed differential gene expression and gene ontology analysis. We found that during late gestation there was a significant over representation of genes involved in inflammation, which is an important component of the implantation pathway of eutherians. We also identified several key genes involved in eutherian implantation that were highly expressed in late pregnant uterine tissue. Using immunohistochemistry we show that these changes occur in the uterine epithelium, and hence at the maternal-fetal interface. Together our data suggest that the late gestation phase of pregnancy in opossums is homologous to the implantation phase of pregnancy in eutherian mammals. Further, we suggest that the processes that facilitate implantation in eutherians, may have had fundamentally different roles when they were co-opted to pregnancy in early viviparous mammals.