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Birth
Birth, also known as parturition, is the act or process of bearing or bringing forth offspring.〔"birth". OED Online. June 2013. Oxford University Press. (Entry 19395 ) (accessed 30 August 2013).〕 In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it is ready to feed and breathe. In some species the offspring is precocial and can move around almost immediately after birth but in others it is altricial and completely dependent on parenting. In marsupials, the fetus is born at a very immature stage after a short gestational period and develops further in its mother's pouch.
It is not only mammals that give birth. Some reptiles, amphibians, fish and invertebrates carry their developing young inside them. Some of these are ovoviviparous, with the eggs being hatched inside the mother's body, and others are viviparous, with the embryo developing inside her body, as in mammals.
==Birth in mammals==
Large mammals, such as primates, cattle, horses, some antelopes, giraffes, hippopotamuses, rhinoceroses, elephants, seals, whales, dolphins, and porpoises, generally are pregnant with one offspring at a time; although, they may have twin or multiple births on occasion. In these large animals, the birth process is similar to that of a human though in most, the offspring is precocial. This means that it is born in a more advanced state than a human baby and is able to stand, walk and run (or swim in the case of an aquatic mammal) shortly after birth.〔 In the case of whales, dolphins and porpoises, the single calf is normally born tail first which minimises the risk of drowning.〔Mark Simmonds, ''Whales and Dolphins of the World'', New Holland Publishers (2007), Ch. 1, p. 32 ISBN 1845378202.〕 The mother encourages the newborn calf to rise to the surface of the water to breathe.
Most smaller mammals have multiple births, producing litters of young which may number twelve or more. In these animals, each fetus is surrounded by its own amniotic sac and has a separate placenta. This separates from the wall of the uterus during labor and the fetus works its way towards the birth canal.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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