Pregnancy is an important biological innovation that has evolved convergently hundreds of times in fish, mammals, reptiles, amphibians, and invertebrates. While many viviparous lineages nourish developing embryos from egg yolk alone, complex placentae that transport large quantities of organic nutrients to the fetus (known as placentotrophy) have evolved independently at least seven times in amniotes (at least once in therian mammals and six times in scincid lizards). The placenta has arisen via homologous associations of the same extraembryonic membranes and uterine tissues in each lineage. Analogous placentae have also evolved multiple times in sharks and teleost fish (anamniotes), but from different ancestral structures, because their embryos lack the amniote-specific extraembryonic membranes. We are currently combining morphological, transcriptomic and proteomic approaches to determine how placentae support embryonic development in lizards, sharks, and mammals. This work is shedding light on the fundamental biology of the placenta in poorly studied species, for example, in determining the mechanisms underpinning nutrient transport in a placental shark that allow embryonic mass increase of almost 3,000 % over 4.5 months of development. Our research is also contributing to new knowledge in evolutionary biology, as we determine whether the same genes have been recruited to support placental function across diverse animals.