Placental is mainly the adjective of placenta: relating to the placenta, or having one. Doctors use it for problems and features of that organ — placental blood, placental tissue, and conditions like placental abruption, where the placenta comes away from the wall of the womb too early. The second use is in biology, and it is the one most learners meet first. Placental mammals are the huge group of mammals whose babies grow inside the mother, fed by a placenta, until they are fairly developed — humans, dogs, whales, elephants, mice. They are set against marsupials like kangaroos, whose babies are born tiny and finish growing in a pouch, and against the odd egg-laying mammals like the platypus. In that biology sense the word can also be a noun: 'a placental' simply means a placental mammal, and you will see 'the placentals' in textbooks. That noun use is quite technical and you never need to produce it yourself — but recognising it saves confusion when you meet it. The word is neutral and scientific; it has no everyday emotional colour at all.