Gestation is the act of an embryo or fetus developing within the body of a female host, usually the mother, in a viviparous animal, like a mammal. The gestation period in humans is equal to the amount of time spent in gestation, plus two weeks. In other animals the gestation period may or may not include an extra period of time, depending on how the period is calculated. A gestation period can differ wildly between species, and even within a single species there may be some range.

For example, the average gestation period is 38 weeks. Gestation may normally last anywhere from 37 to 42 weeks, however, and be considered well within the range of a healthy gestation period. Any child born before 37 weeks of gestation is considered preterm, and this can be accompanied by any number of health risks. In fact, preterm is one of the leading causes of infant mortality, with around one-in-six infant deaths the result of a preterm birth. Longer than a 42 week gestation period is considered post-term, although in most cases in the modern world labor will be induced or an operation will be undertaken before 42 weeks is passed.

The length of a species’ gestation period depends on a number of factors, including the size of the animal, its metabolism, and how exactly it bears its young. For example, placental mammals tend to have the longest gestation periods, as their young are birthed in an advanced state of development. Marsupial mammals, on the other hand, have much shorter gestation periods, since their young can still be quite unformed, since they have a protective pouch to continue to develop within in relative safety.

For example, while a human has an average gestation period of 266 days, a much smaller mammal, like a chipmunk, can have a much shorter gestation period, in this case 31 days. Comparably-sized placental mammals, like grizzly bears, have comparable gestation period lengths, in this case 225 days. Within similar animals there is some variation as well: grizzly bears have an average gestation period of 225 days, but black bears have an average of 210 days, and polar bears have an average of 240 days.

Larger animals, which require more time to develop before being birthed, often have longer gestation periods than humans. Animals that are born in a more fully-functional state than humans may also have longer gestation periods. For example, horses have a gestation period of around 335 days, but when born they can stand almost immediately and are quickly functional. Giraffes, which are sizable animals, have an average gestation period of 435 days, rhinoceroses have an average period of 450 days, and elephants, the largest land mammals, have an average period of 645 days.

Other animals also have a gestation period, and the term may be used for the incubation period of eggs for egg-laying animals like birds. Incubation periods tend to be quite a bit shorter than the gestation periods of comparably-sized mammals. A chicken, for example, has an incubation period of 20 days, and even larger birds like emus have incubation periods of only around 55 days.