Gestation
2 of the 6 encyclopedias on this shelf carry an entry for Gestation. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
in physiology, the act of carrying young in the uterus from the time of conception to that of parturition. The average time of a woman's pregnancy is 9 solar months, or about 280 days, though it may be as few as 7 or as many as 10. The period of gestation is shorter in carnivorous than in herbivorous animals . The young of the former are also less developed at birth, their eyes not opening for several days thereafter. Herbivorous animals: The elephant has 20 or 21 months ' gestation; the giraffe, 14 months ; dromedary, 12 months; buffalo, 12 months; ass, 12 months; nths; mare, upwards of 11 months; rhinoceros, 9 months ; cow, 9 months; many of the larger deer, over 8 months; sheep and goat, 5 months ; pig, 4 months . Rodents: Beaver, 4 months; dormouse, 31 days; rabbit , 30 to 31 days; squirrel and rat, 28 days; guinea-pig, 21 days or less. Carnivorous: Bear, 6 months; lion, 108 days; puma, 79 days; fox, wolf, and dog, 62-63 days; cat, 55 or 56 days. Pouched animals: Kangaroo, 39 days; opossum, 26 days. Cetaceous animals: Greenland whale, about 10 months. The most common duration for the varieties of monkeys is 7 months. Oviparous animals: The goose sits 30 days; swan, 42 days; hens, 21 days; ducks, 30 days; pea-hens and turkeys, 28 days; canaries, 14 days; pigeons, 21 days; parrots, 40 days. The periods are subject to considerable variation, especially in domestic animals, and various conditions modify the period, of which the above are only the averages.
Aiton's Encyclopedia (1910)
in physiology, the carrying of young before birth. In a general way the period of gestation is longer for the larger animals. The period is short for young that come blind or otherwise immature, as in the case of kittens. The following is a list of a few animals and the average or usual period of gestation for each: Elephant 2 years Giraffe 14 months Dromedary 12 months Buffalo 11 months Mare 11 months Cow 9 months Deer 8 months Monkey 7 months Bear 6 months Sheep and goat 5 months Sow 4 months Beaver 4 months Lion 3 1/2 months Dog, wolf, and fox 2 months Cat 50 days Kangaroo 39 days Rabbit 30 days Squirrel 28 days Rat 28 days Opossum 26 days
What Has Changed in the Last Century
Most of the figures Collier's printed in 1921 and Aiton's in 1910 would still pass a veterinary textbook: the elephant's 20-22 months, the mare's 11, the cow's nine, the cat's 55-odd days. Two entries have been corrected — the Virginia opossum's gestation is about 13 days, not the 26 both books give, making it among the shortest of any mammal, and marsupials are now known to practise embryonic diapause, holding a fertilised egg in suspension for months until conditions suit. Human pregnancy is still reckoned at about 280 days, but the count is now standardised from the last menstrual period (roughly 266 days from conception) and confirmed by ultrasound dating rather than arithmetic. The biggest practical change is at the short end: Collier's notes pregnancies "as few as 7" months, and infants born that early, once rarely saved, now survive routinely in neonatal intensive care, with some surviving from 22 weeks.
Written for this edition, 2026.