The term “gestation period” refers to the process in mammals during which the fetus develops inside the mother. Gestation begins at conception and ends at birth. The gestation period is different for all mammals. For humans, it is nine months (266 days). Gestation periods can be as low as 16-23 days (hamsters), or as long as 645 days for African elephants. Other notable gestation periods include: giraffes (420-450 days), sperm whales (480-590 days), rhinoceroses (450 days), mice (19-21 days), opossums (12-13 days), and kangaroos (42 days).
Most of the time, the length of the gestational period is relative to the size of the mammal. In other words, larger animals have longer gestational periods. However, this is not always the case. For example, in animals that have a breeding season, where the animals give birth during specific times of the year, live births occur when food is abundant. It is the amount that the fetus grows during the gestation period that determines its length.
In mammals, pregnancy is determined to occur when a fertilized zygote implants in the uterus, and ends with birth. In humans, pregnancy is divided into three trimesters, each approximately three months long. Live birth is known by the scientific term viviparity. Ovipary is the term used for animals that lay eggs.