Marsupials are often compared with Placental Mammals (Placentalia). But one of the biggest difference between these 2 animals is their reproductive systems. The marsupial's reproductive system differs so much from the placental mammals.

One of the differences is that the female marsupial has 2 lateral vaginas. These 2 lateral vaginas leads to 2 separate uteruses and these uteruses both open externally through the same orifice (opening). A median vagina or a third canal is used for giving birth. This third canal can be either transitory or permanent.

Another difference is that the male marsupials have 2 penises. The male marsupial's 2 penises corresponds to the females' 2 vaginas. The male's penises are on used for discharging semen in the female's vaginas. Because male marsupials have a uro-genital sac which is used to store their wastes before excreting it.

A yolk sac develops inside a pregnant female marsupial's womb. The yolk sac helps deliver nutrients or nourishment to the developing embryo. Female marsupials give birth at a very early stage of the embryo's development. They give birth after about 4-5 weeks of pregnancy. After birth, newborn marsupials have to crawl up to their mother's body and attach themselves to a nipple. This move is vital for the newborn marsupial's survival. Because a newborn marsupial is literally only bean-sized. And it will continually grow and develop by attaching themselves to their mother's nipple and continuously suck on it for food and nourishment. The mother's nipple is located on it's underside It's either inside or outside the pouch (marsupium).

This move which is vital to a newborn marsupial's survival needs great strength. One thing about newborn marsupials being only bean-sized is that the development of their forelimbs and facial structure is quite fast than placental mammals. This enables them to accomplish the task of attaching themselves to their mother's nipple. But there's one disadvantage to this. Their hind limbs and  brain structure has slower development. 

Once the newborn marsupials are in the mother's pouch, they stay their for a couple of weeks without going out. But after some time, when they developed enough, they're able to leave the mother's pouch or marsupium for short periods of time. After that, they return inside their mother's pouch for nourishment, warmth and safety or protection.