In marsupials, ova are shed by both ovaries into a doublehorned or bicornate uterus. The developing embryos remain in the uterus for 12–28 days, and most of the nourishment comes from an energy sac attached to the egg (yolk sac).
There is no placenta (except for one groups of marsupials, the bandicoots, that have an interchange surface that resembles a true placenta). Gestation is thus short (less than one month), and much of the development of the young will occur outside of the female reproductive tract.
At birth, the offspring are extremely altricial (poorly developed). In many marsupials such as kangaroos, offspring will migrate to the nipples where they attach. This will ensure that they remain in contact with their nourishment sources, and also probably serve to secure the young and prevent them from falling off the mother at an age when they do not have the strength to hold on by themselves.
In red kangaroo (Macropus rufus), the largest of all marsupials, young climb unaided to the pouch within a few minutes of birth, remain on the nipple for 70 days, protrude from the pouch at 150 days, emerge on occasion at 190 days, and permanently leave the pouch at 235 days, but is fully weaned only after a year.
Marsupials have the tiniest young in relation to adult size. In the eastern gray kangaroo (Macropus giganteus), young at birth weigh less than 0.0001% of the female mass. Put into context, this would be similar to a 150 lb (68 kg) human female giving birth to individual babies that would each weigh 0.0048 lbs (22 g), or 0.08 oz. But extreme altriciality is not a disadvantage in evolutionary terms.
In fact, many scientists believe that this is instead a great advantage as the small investment in each neonate allows females to minimize investment in young and be more flexible and responsive to environmental conditions.
Marsupial mammal development
Mechanistically, if environmental conditions become too tough to raise young successfully, starvation would terminate the production of milk and lead to rapid death of young, thereby saving energy lost (versus placental mammals that have a greater energy investment). This would give marsupial females a competitive advantage over animals with internal pregnancy (placental mammals) in unpredictable environments.
Some marsupials such as the eastern gray kangaroo also display other reproductive oddities. Pregnancy following copulation does not affect the cycle and the female can become pregnant again as her first young (litter size usually is one) move to the pouch. The second fertilized egg undergoes diapause (halt in development) until the first young either reaches adulthood or dies. In this case, the diapause is facultative or changes length depending on circumstances such as food availability or season. Then, the second egg immediately resumes development so that birth occurs as soon as the mother’s pouch is available. So at any one time, the female can have three young: one in the placenta, one in the pouch, and one joey out of the pouch and still suckling.