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How do marsupials differ from placental mammals in the development of their young?

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Red kangaroo (Macropus rufus) — marsupial Red kangaroo (*Macropus rufus*). The home range of this species spans much of Australia's interior, and it is the largest member of the family Macropodidae.

Marsupials and placental mammals differ significantly in their reproductive strategies and the development of their young:

Intrauterine Development: Placental mammals undergo a prolonged period of development within the uterus, where the placenta facilitates the exchange of nutrients and waste between the mother and the fetus. In contrast, marsupials have a very short gestation period—ranging from about 8 to 40 days—because their uterine walls are not specialized for long-term embryo implantation.123
Birth and Early Development: Marsupials give birth to young in a highly vulnerable, embryonic state. After birth, these newborns must migrate to the mother's teats to continue their development.342
Postnatal Care: Many marsupials utilize a marsupium, or pouch, which provides a protected environment for the young to remain attached to the teats for weeks or months until they are sufficiently developed to survive independently. Placental mammals, having completed this developmental phase in the womb, are born at a much more advanced stage.351

While these differences are sometimes viewed as primitive versus advanced, they are more accurately understood as distinct evolutionary adaptations to different environmental conditions.3