Gestation Periods


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•  The average 38 week pregnancy in humans can be positioned on a graph showing the correlation between

    animal size and the development of the young at birth for other mammals

For mammals, the gestation period is the time taken for a foetus to develop – beginning with fertilization and ending with birth

  • The duration of the gestation period will differ markedly between different species of animal

Generally, there are two main factors that contribute to the length of the gestation period:

  • Animal size / mass – larger animals tend to have longer gestation periods (as they tend to produce larger offspring)
  • The level of development at birth – more developed infants will typically require a longer gestation period

The level of development at birth for mammalian infants can be described as either atricial or precocial

  • Altricial mammals give birth to relatively helpless, undeveloped offspring that need extended rearing
  • Precocial mammals give birth to more developed offspring that are mobile and independent and require minimal rearing
  • Generally, altricial mammals (e.g. marsupials and rodents) require shorter gestation periods than precocial mammals (e.g. ungulates such as cows, pigs and rhinoceroses)

While the length of a gestation period does appear to positively correlate with size and development, other factors also exist

  • Some mammal species may have similar gestation periods despite having significantly different body masses

Length of Gestation versus Mammalian Mass 

gestation vs mass