Marsupials are mammals in which the female typically has a pouch in which it rears its young through early infancy. They differ from placental mammals in their reproductive traits. The pregnant female develops a kind of yolk sack in her womb which delivers nutrients to the embryo. The embryo is born at a very early stage of development, upon which it crawls up its mother's belly and attaches itself to a nipple (which is located inside the pouch). It remains attached to the nipple for a number of weeks. The offspring later passes through a stage where it temporarily leaves the pouch, returning for warmth and nourishment. Most marsupials tend to be slow moving creatures, but some species of kangaroo can reach speeds of up to 50 km/h (31 mph).