The animal with the shortest gestation period is the Virginia opossum, with an average of 12 days. However, opossums are marsupials and give birth to partially developed young. Among animals that give birth to developed young, the golden hamster also has a short gestation period: 14 to 18 days long.
Marsupials tend to have very short gestation periods for animals of their size because development of their young continues after birth inside the mother's pouch. The babies are little more than embryos when born and are incapable of surviving outside the pouch. They latch on to the mother's nipples for nourishment and become semi-permanently attached until they have grown enough to survive outside.
Rodents also have very short gestation periods; however, their young are born fully formed. Golden hamsters not only have very short pregnancies, but their young are capable of becoming pregnant just 4 weeks after being born themselves.