Understanding the evolution of complex traits is of fundamental interest to scientists and non-scientists alike. The placenta is an excellent example of a complex trait that has evolved repeatedly throughout the animal kingdom. This repeated evolution and ongoing elaboration appears to point towards an adaptive advantage to specific environmental conditions; however, this potential benefit is currently insufficiently understood. In this thesis, I attempt to shed light on the causes and consequences of placental evolution by drawing on insights from placental live-bearing fish. I show that the placenta evolves in high ‘performance-demanding’ environments by reducing the reproductive burden of females during pregnancy thereby improving their swimming performance. However, the intimate link the placenta forms between mother and fetus also poses a risk: maternal exposure to adverse environmental conditions (e.g. malnutrition, parasite infestation) is likely to have unfavorable consequences for fetal development.