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Reproduction
Marsupial reproductive organs differ from those of other animals. Female marsupials have two vaginae, each with its own cervix and uterus. However, they only have two ovaries, as humans do. One ovary leads to each uterus. The marsupial ovulation is similar to that of other mammals. One egg is usually released in each cycle. Male marsupials usually have a pronged penis,which is drawn into the male's body when not in use. After mating, conception takes place in one of the female's vaginae. The two vaginae fuse together when the female gives birth to create a single passage for birth to take place. Whether or not the organs separate again depends on the species of marsupial.
Gestation
Marsupials have much shorter gestation periods than other mammals. Although the female marsupial gives birth to live young, the tiny animals are only partially developed. Newborn marsupials have strong front limbs with claws that help them to crawl into their mothers pouch and toward her nipples. Marsupials have short gestation periods as nourishment is not supplied to the placenta continually as it is in other mammals. Instead, the marsupial placenta has a limited supply of nourishment. Young marsupials need the nourishment of their mother's milk to complete their development.
The Pouch
The nature of a marsupial's pouch varies between species. Marsupials that rear one young animal at a time usually have a large pouch that can cover the young marsupial completely. The kangaroo is an example of this. Marsupials that raise a number of young at once may have a pouch that is simply a flap of skin around the area that the young animals feed from. These tiny, partially developed marsupials remain attached to their mother's nipple until they have developed enough be left alone in a nest. The combined weight of the young becomes too great for the mother to carry them around in a pouch for as long as marsupials with one young do. An example of this type of pouch can be seen in the opossum.
Lactation
Once a marsupial has given birth to its young, the embryo attaches to its mother's nipple to continue its development. Depending on the species, the lactation stage can last for as little as a few weeks or as long as 18 months. A female marsupial produces three stages of milk during lactation. The first is rich in antibodies, and the second and third types of milk contain a high level of the proteins, carbohydrates and lipid needed for development. The third type of milk is produced to supplement the diet once the young marsupial has started to consume solids. The young marsupial will become weaned off the third stage of lactation, ready to become independent.