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妊娠
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胚胎与胎儿发育
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胎儿
术语胎儿
(fetus，自拉丁语：后代)定义为生命第3月致诞生之月的生命发育有机体。在这个时期，它从约2.54厘米（1英寸）长和一盎司重发育成约50.8厘米(20英寸)长与约3.18千克(7磅)重的宝宝。在这个发育时期的第1周里，男女的内部性器官开始明显出现分化。不久，外部性器官发育特征显现。大约于5个月时，胎儿活动变得如此强而有力，以至于翘首以待的妈妈也能够感受得到。从前，人们认为这个称之为胎动的现象为生命进入身体的时候。
在胎儿整个生长发育期间，他/她漂浮在羊水里而得到了很好的保护。在6个月末，胎儿长约15.24厘米(6英寸)、重约0.68千克（1.5磅）。此时控制呼吸的脑中枢神经开始发育。这样的胎儿早产（尽管可能性非常之小）后实际上要生存下来简直是不可能的。因为未有呼吸而仍然导致脑损伤的可能性非常大。胎儿的最后数月发育为的是进一步的完善结构与功能，如脑的体温控制机制的完善与皮下脂肪保护层的增厚等等。在男胎，睾丸下降到阴囊。如果未下降，出生后必须在合适时间里做手术矫正。不然，将会导致男性不育。
出生前的最后数周，胎儿不仅在个头上快速地生长，而且其体重也增加了许多。胎儿出生时体重小于2.27千克(5磅)者，则为早产儿。 |
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Pregnancy
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Development of Embryo and Fetus
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The Fetus
The word fetus (Latin: offspring) is used to describe the growing organism from the beginning of the third month of its life to the moment of birth. During this time, it develops from a small growth of slightly over an inch weighing only a fraction of an ounce into a baby of about twenty inches in length weighing approximately seven pounds. In the first weeks of this development, the male female sex differentiation becomes apparent in the internal sex organs. A little later, the external sex organs develop their characteristic structure. Sometime around the fifth month, the fetal movements become strong enough to be felt by the expectant mother. This so called quickening was formerly believed to be the moment when life entered the new body.
Throughout its growth, the fetus is well protected from injuries as it floats almost weightlessly inside a fluid filled sac called the amniotic sac. At the end of the sixth month, the fetus measures about six inches in length and weighs about one and a half pounds. At this time, the centers of the brain which control breathing begin to develop. It is not entirely impossible (although very unlikely) that such a fetus could actually survive a premature birth. However, the probability of brain damage because of still ineffective breathing is great. The last months of fetal development bring further refinements, such as the temperature control mechanism in the brain and a protective layer of fat under the skin. In the case of a male fetus, the testicles descend into the scrotum. If this descent should fail to occur, corrective measures have to be taken sometime after birth. Otherwise, sterility will result.
During the final weeks before birth, the fetus not only grows rapidly in size, but also gains much of its weight. The birth of a fetus weighing less than five pounds is premature. |
