According to TreeHuggery.com https://www.treehugger.com/animals-with-the-longest-gestation-period-4869368, elephants have the longest gestation period of all mammals. Since elephants are the largest living and biggest-brained land animal in the world, there’s a lot of development for elephants to do in the womb.
The average gestation period of an elephant is about 640 to 660 days, or roughly 95 weeks. By comparison, a human pregnancy lasts an average of 280 days, or 40 weeks. Female elephants live for 60 to 70 years, but only have about four offspring throughout their lifetime.