In placental mammals, such as humans, the placenta allows the fetus to develop in the mother's womb. Because of the incomplete placenta, marsupials are unable to support fetuses in the uterus for an extended period of time. Thus, the infants are born embryonic in size and development. These newborns then must make a perilous journey, without the aid of mom, to the safe haven of the pouch. Once in the pouch, the infants attach themselves to a nipple.
Infants remain in the pouch for weeks, growing and developing.
To ensure that the newborn does not slip off the nipple, the infant's lips are partially sealed, leaving a circular opening for the mother's nipple. Once the baby begins suckling, the nipple expands in the infant's mouth. Generally speaking, as soon as marsupial infants are born, the mother becomes pregnant again. But as long as suckling young are present, the embryos do not develop. When the young begin leaving the pouch, the embryos resume development and are born soon after.