Which mammal has the longest pregnancy?

An elephant carries its calf for roughly twenty-two months, close to two years and longer than any other mammal. The length is driven by the brain. Elephants are born with a large, highly developed brain and must be able to stand, walk and keep up with a moving herd within hours, in open country where a helpless newborn would not survive. Building that before birth takes time, and the calf still spends years learning from the herd afterwards. The cost is that reproduction is very slow: a female produces one calf at a time, at intervals of several years, and nurses it for a long stretch, which is a large part of why elephant populations recover so slowly from losses. Long gestation is generally the pattern for large, long-lived mammals with well-developed young, while small animals with short lives and helpless litters gestate for weeks.

Elephant calves are cared for by several females in the herd, not the mother alone.