For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species.
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. For example, a cat's gestation normally takes 58–65 days while an elephant's takes 645 days. 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. In dogs there's a positive correlation between gestation time and a small litter size.
Gestation periods 
See also 
- ^ "Gestation period". McGraw-Hill's AccessScience Encyclopedia of Science & Technology Online. McGraw-Hill.
- ^ a b c "gestation". Britannica - The Online Encyclopedia. Encyclopædia Britannica.
- ^ "Average Animal Gestation Periods and Incubation Times". MSN Encarta. Microsoft. Archived from the original on 2009-10-31.
- ^ "Does gestation vary by ethnic group? A London-based study of over 122000 pregnancies with spontaneous onset of labour". International Journal of Epidemiology.
- ^ Paris Hill Farm. "Alpacas 101". Retrieved 2012-07-14.
- ^ Northwest Alpacas. "Alpaca Q&A". Retrieved 2012-07-14.
- ^ University of Michigan. "Ursus Americanus". Retrieved 2013-01-16.
- ^ a b c d e f g Anamnese en lichamelijk onderzoek bij gezelschapsdieren, A.Rijnberk, F.J.van Sluis, 2nd print, Bohn Stafleu van Loghum, 2005, (Current Dutch veterinary examination study book for small domestic mammals)
- ^ The Laboratory Rat , 2nd Edition, Eds. Mark A. Sucklow, Steven H. Weisbroth, and Craig L. Franklin. Page 151.