Which mammal has the longest pregnancy?

An African elephant pregnancy lasts around 22 months, roughly 650 days, and Asian elephants are only slightly shorter. Scientists think the long gestation helps the calf's large brain develop, so it can walk and follow the herd soon after birth. A newborn calf usually weighs around 100 kg and can stand within an hour or so. Elephants usually have a single calf, and females typically give birth every four to five years. Some non-mammals may have even longer pregnancies, such as the frilled shark, which is thought to carry its young for more than three years.