Opossums are North America's only marsupial so unlike other mammals, they don't have their babies in a den or nest. They carry them around in their pouch. Opossums have 13 nipples in their pouch so they can carry as many as 13 babies, though we usually see mothers with 9 to 11 in this area. Those babies will not be weaned until they are about 2 1/2 months old, then the mothers will still carry all those babies around on her back for another month or two before they go off on their own. Then she will immediately get pregnant again and repeat the cycle so she can squeeze in two litters every summer.
Opossums are very good mothers, and very prolific.