Which mammal has the shortest gestation period?

Virginia OpossumThe shortest known gestation period is 12 to 13 days. This record is shared by three marsupials, which is kind of an unfair comparison to all other mammals as in marsupials, the young are born immature and have to continue developing in a pouch on the mother. The three mammals with this very quick gestation period are:  the American or Virginian opossum (Didelphis marsupialis); the rare water opossum, or yapok (Chironectes minimus), of central and northern South America; and the eastern native cat (Dasyurus viverrinus) of Australia.

More about the Virginia Opossum [source: Wikipedia]

It is the largest member of its genus, family and order and is the largest of the opossums. They are typically 15–20 inches (38–51 cm) long and weigh between 9 and 13 pounds (4–6 kg). Their coats are a dull grayish brown, other than on their faces, which are white. Opossums have long, hairless, prehensile tails, which can be used to grab branches and carry small objects. They also have hairless ears and a long, flat nose. Opossums have 50 teeth and opposable, clawless thumbs on their rear limbs.