The infraclass of therian mammal species in which females produce a placenta to connect the fetus to the uterus. This enables nutrients and oxygen to get to the fetus and provides a means of eliminating waste products. As a result, placental mammal or eutherian mammals can carry their young within the uterus until late in fetal development. This has a selective advantage because it results in decreased infant mortality. Included in this infraclass are dogs, cats, bears, whales, monkeys, humans, and many other mammals.