Pregnancy female opossum lasts only 11 to 13 days. Babies are born so small, that has not yet adapted to an independent life. Although the opossum resembles a rodent and, in fact, this animal is a marsupial.
A couple dozen newborn opossums crawl into the mother's bag and continue to develop it, as long as they do not become too crowded. Then they go out and traveling with her mother, clinging to the hair on the back.