Animals that give birth to live young are called viviparous animals. In viviparous animals, the mother's egg is fertilized by the father's sperm inside her body and, after a brief gestational period, the young animal is born.
Mammals and marsupials are animals that give birth to live young. Marsupials are different in that their young are born alive, largely unformed at birth, and finish developing after birth. Kangaroos and koalas are examples of marsupials.
Birth, also known as parturition, is the process in which a live, fully developed baby comes out of the mother's uterus. Mammals are the only species on Earth that feed their babies milk from the mother's body. Most mammals have a placenta that feed the young while they are still in the mother's uterus.
After the baby is born, the placenta, the organ that feeds the baby in the uterus, is expelled from the mother's body.