Kangaroos usually have one young annually. The young kangaroo, or joey, is born alive at a very immature stage, when it is only about 2 cm long and weighs less than a gram. Immediately after birth it crawls up the mother's body and enters the pouch. The baby attaches its mouth to one of four teats, which then enlarges to hold the young animal in place. After several weeks, the joey becomes more active and gradually spends more and more time outside the pouch, which it leaves completely between 7 and 10 months of age.