Gestation period

(embryology) The period in mammals from fertilization to birth.

In mammals, the interval between fertilization and birth. It covers the total period of development of the offspring, which consists of a preimplantation phase (from fertilization to implantation in the mother's womb), an embryonic phase (from implantation to the formation of recognizable organs), and a fetal phase (from organ formation to birth).

There is widespread confusion over the duration of the gestation period in humans because of the way in which it is defined medically. The time of ovulation, and hence the time of fertilization, is difficult to determine in humans, so for purely practical reasons doctors measure the duration of pregnancy as the interval between the last menstrual period and birth, which is typically about 40 weeks or 280 days. For comparison with other mammals, however, the true gestation period between fertilization and birth in humans is about 267 days.

The length of the gestation period in mammals depends primarily on body size and the state of development of the offspring at birth. Large-bodied mothers have big offspring that take longer to develop, and development is also prolonged for offspring that are born at an advanced stage of development. Compared to all other mammals, human beings are found to have one of the longest gestation periods relative to body size.

One remarkable feature of mammalian gestation periods is that they show very little variability within a species. After excluding exceptional cases, departures from the average usually lie in a range of no more than ±4%. This is one of the smallest degrees of variability found in any biological dimension. See also Pregnancy; Reproductive system.

The noun has one meaning:

Meaning #1: the period during which an embryo develops (about 266 days in humans)
  Synonym: gestation

For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at the embryo stage. ^[1] The duration of this period varies between species.

Contents 1 Duration 1.1 Gestation periods 2 See also 3 References 4 Notes

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. ^[2] For example, a cat's gestation normally takes 58–65 days while an elephant's gestation takes 645 days. ^[3] 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.^[2]

Various other factors can come into play in determining the duration of gestation. For humans, males normally gestate several days longer than females and multiple pregnancies gestate for a shorter period.^[2] Ethnicity may also lengthen or shorten gestation.^[4] Some events, such as preterm birth, can greatly shorten the length of gestation.

Gestation periods

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Animal	Gestation period (days)
Mammals
baboon	187
bear (black)	210
bear (grizzly)	225
bear (polar)	240
beaver	122
buffalo (American)	270
camel (Bactrian)	~410
cat (domestic)	58–65
chimpanzee	230–250
chinchilla	110–120
chipmunk	31
cow	279–292
deer (white-tailed)	201
dog (domestic)	58–70
donkey	365
elephant (Asian)	645
elk (Wapiti)	240–250
fox (red)	52
giraffe	420–450
goat (domestic)	145–155
gorilla	257
guinea pig	68
hippopotamus	225–250
horse	330–342
human	254-294
kangaroo	42
leopard	92–95
lion	108
llama	330
mink	40–75
monkey (rhesus)	164
moose	240–250
mouse (domestic white)	19
mouse (meadow)	21
muskrat	28–30
opossum (American)	12–13
otter	270–300
pig (domestic)	112–115
porcupine	112
puma	90
rabbit (domestic)	30–35
raccoon	63
rhinoceros (black)	450
seal	330
sea lion (California)	350
sheep (domestic)	144–151
squirrel (gray)	30–40
tiger	105–113
whale (sperm)	480–500
wolf	60–68
wombat	26–28
zebra (Grant's)	365
Birds
chicken	20–22
duck	26–28
finch	11–14
goose	25–28
ostrich	35-45
parrot	17–31
pheasant	24
pigeon	10–18
quail	21–23
swan	33–36
turkey	28

See also

• mammalian pregnancy
• human pregnancy

References

• Gestation, Incubation, and Longevity of Selected Animals
• David Crystal, The Cambridge Factfinder Cambridge: Cambridge University Press, 1998 (84).

Notes

1. ^ "Gestation period". McGraw-Hill's AccessScience Encyclopedia of Science & Technology Online. McGraw-Hill. http://www.accessscience.com/abstract.aspx?id=288400&referURL=http%3a%2f%2fwww.accessscience.com%2fcontent.aspx%3fid%3d288400. 
2. ^ ^{a b c} "gestation". Britannica - The Online Encyclopedia. Encyclopedia Britannica. http://www.britannica.com/EBchecked/topic/232124/gestation. 
3. ^ "Average Animal Gestation Periods and Incubation Times". MSN Encarta. Microsoft. Archived from the original on 2009-10-31. http://www.webcitation.org/5kwpsKDKF. 
4. ^ "Does gestation vary by ethnic group? A London-based study of over 122000 pregnancies with spontaneous onset of labour". International Journal of Epidemiology. http://ije.oxfordjournals.org/cgi/content/full/33/1/107. 

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