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Gestation period
For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth.〔(【引用サイトリンク】title=Gestation period )〕 The duration of this period varies between species.
==Duration==
For most species, the amount a fetus grows before birth determines the length of the gestation period. Smaller species normally have a shorter gestation period than larger animals.〔(【引用サイトリンク】title=gestation )〕 For example, a cat's gestation normally takes 58–65 days while an elephant's takes nearly 2 years (21 months). However, growth does not necessarily determine the length of gestation for all species, especially for those with a breeding season. Species that use a breeding season usually give birth during a specific time of year when food is available.〔
Various other factors can come into play in determining the duration of gestation. For humans, male fetuses normally gestate several days longer than females and multiple pregnancies gestate for a shorter period.〔 Ethnicity may also lengthen or shorten gestation.〔(【引用サイトリンク】title=Does gestation vary by ethnic group? A London-based study of over 122000 pregnancies with spontaneous of labour )〕 In dogs there's a positive correlation between gestation time and a small litter size.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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