As God would have it, Kangaroos have one of the easiest labors of all mammals, they are only literally pregnant for just twenty eight days, because they have no placentas, their baby can't stay in their stomach, so they easily excrete the pregnancy out, and then they put it in their pouch pocket.
There baby kangaroo is called a "joey" which is so small at this point, its eyes, ears, organs and central nervous system aren’t completely developed. About the only thing it has going for it, aside from the claws, is the olfactory bulb, a section of the brain devoted to the sense of smell. This allows the baby roo to follow the scent of saliva into the pouch pocket.
Kangaroos usually have one young annually. The young kangaroo, or joey, is born 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.
After three months they will start to stick their heads out. Three months later and they can start getting out of the pouch for small bits of time. Three months after that and most of the time is spent out of the pouch. The joey will still spend time near its mom once it is too big for the pouch as well.