There's a joke from my favorite show "Arrested Development" where Buster, the mama's boy character, was in his mother's uterus for 11 months and that by the end of gestation there were, as the title says, clawmarks on the uterus.
It's funny stuff if you've ever watched "Arrested Development," but after doing a little research it turns out that 11 months isn't anywhere near the longest gestation period for animals.
Human babies stay inside their mothers' bellies for between 253 and 303 days (on average, obviously), and the animals with the closest gestation period to humans are horses, deer, and cows, all of which carry babies for about nine months.
Some animals go even longer, though. Donkeys carry babies for a year, while a whale fetus can gestate from 365-547 days. Some elephant babies stay locked up for almost two years!
For some species, the gestation period is significantly shorter than nine months. Bear babies gestate for 5-7 months, and pigs and lions go just over 100 days. I know a lot of human ladies that would REALLY appreciate it if pregnancy lasted 3 1/2 months! Though I don't think they'd appreciate birthing a pig and/or lion.
And now you know.