Gestation period is the time taken for embryo development after fertilization of egg in the female animal.

In humans the gestation period is approximately 38 weeks. The gestation periods of chimpanzees (237 days), gorilla (257 days) and orangutan (260 days) indicate that the approximate nine-month gestation period predates the emergence of human beings as a species.

The gestation periods of other animal reveals, that the length of the gestation period is largely a matter of the animal specie's size. Asian elephants have a gestation period of 645 days and African elephants 640 days. Dogs and other canines have a gestation period of about 60, which is also the period for cats. Even within the species of apes and monkeys gestation period seem to be a matter of size. For Rhesus monkeys it is 164 days and baboons 187 days.

For small animals such as rabbits the period is about 33 days and for mice about 20 days. Thus, it appears that it takes a longer time for a big animal such an elephant to grow a baby than for a smaller animals, such as a mouse.