Gestation is the carrying of an embryo or fetus inside a female viviparous animal, until its birth. Mammals can have more than one embryo or fetus at the same time.

The time between fertilization of an ovum and the birth of the young is called the gestation period. Humans have a gestation period of around 37 weeks or about 9 months and 1 week. Syrian hamsters have a gestation period of 16 days. That is believed to be the shortest gestation period for a mammal. The time the gestation period takes depends on the size of the animal, smaller ones have short lengths and bigger ones have the opposite.