Patrick Abbott and I just published a Insight article in eLife about a recent manuscript that demonstrated similarities between genes expression dynamics in the short-lived marsupial placenta and marsupial mammary glands with genes expressed in eutherian placentas. This illustrates that many developmental functions in marsupials and eutherian mammals are accomplished by different tissues, but similar genes. We argue that similar comparative genomics approaches will yield more insights into the evolution and molecular basis of pregnancy.