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.
Animal gestation calendar, 1917. Dates of service and date of birth for mares, cows, ewes, sow, and variation in gestation periods. Also gives gestation periods for ass, goat, dog, cat, rabbit, Guinea pig, goose, turkey, duck, hen, Guinea hen and pigeon.
^ abcdefAnamnese 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.