In eutherians, the energy invested by the mother in rearing young before birth (via placentation) and after birth (via lactation) is roughly equally. In marsupials, gestation is brief, the placenta forms late in pregnancy, and lactation is extended. Guernsey et al. show that genetic features that regulate development via the placenta in eutherians are shared with the short-lived marsupial placenta (red arrows). They also show that some of the genes that underlie placental functions in eutherians are expressed during lactation in marsupials (blue arrows), including various conserved components of lactation itself (black arrow; Lefèvre et al., 2010). Note: time scales are not absolute.